California regulates motorcycle aftermarket parts that have the potential to impact emissions. In most cases, the sale and use of emissions related aftermarket parts on motorcycles is prohibited unless it is either a "Replacement Part" as defined by California, or is a specifically authorized use of that part as reflected in an Executive Order ("EO Part.")

BARCLAYS OFFICIAL CALIFORNIA CODE OF REGULATIONS
TITLE 13. MOTOR VEHICLES
DIVISION 3. AIR RESOURCES BOARD
CHAPTER 1. MOTOR VEHICLE POLLUTION CONTROL DEVICES
ARTICLE 1. GENERAL PROVISIONS

1900. Definitions.

(1) "Add-on part" means any aftermarket part which is not a modified part or a Replacement Part. (2) "Consolidated part" means a part which is designed to replace a group of original equipment parts and which is functionally identical of those original equipment parts in all respects which in any way affect emissions (including durability).
(3) "Emissions-related part" means any automotive part, which affects any regulated emissions from a motor vehicles which is subject to California or federal emissions standards. This includes, at a minimum, those parts specified in the "Emissions-Related Parts List," adopted by the State Board on November 4, 1977, as last amended May 19, 1981.
(14) "Modified Part" means any aftermarket part intended to replace an original equipment emission-related part and which is not functionally identical to the original equipment part in all respects which in any way affect emissions, excluding a consolidated part.
(20) "Replacement Part" means any aftermarket part intended to replace an original equipment emissions-related part and which is functionally identical to the original equipment part in all respects which in any way affect emissions (including durability), or a consolidated part.

The California Air Resources Board (ARB) regulates aftermarket parts and has promulgated regulations that essentially place all emissions related aftermarket parts into two categories:
§ 2221. Replacement Parts.
(a) Any Replacement Part subject to the provisions of this article shall be presumed to be in compliance with this article unless the executive officer makes a finding to the contrary pursuant to Section 2224(a).
(b) The manufacturer of any replacement part subject to the provisions of this article shall maintain sufficient records, such as performance specifications, test data, or other information, to substantiate that such a replacement part is in compliance with this article. Such records shall be open for reasonable inspection by the executive officer or his/her representative. All such records shall be maintained for four years from the year of manufacture of the replacement part.
§ 2222. Add-On Parts and Modified Parts.
(a) As used in this section, the terms "advertise" and "advertisement" include, but are not limited to, any notice, announcement, information, publication, catalog, listing for sale, or other statement concerning a product or service communicated to the public for the purpose of furthering the sale of the product or service.
(b)(1) Except for publishers as provided in subsection 3, no person or company doing business solely in California or advertising only in California shall advertise any device, apparatus, or mechanism which alters or modifies the original design or performance of any required motor vehicle pollution control device or system unless such part, apparatus, or mechanism has been exempted from Vehicle Code section 27156, and the limitations of the exemption, if any, are contained within the advertisement in type size to give reasonable notice of such limitations.
(2) Except for publishers as provided in subsection 3, no person or company doing business in interstate commerce shall advertise in California any device, apparatus, or mechanism which alters or modifies the original design or performance of any required motor vehicle pollution control device or system and not exempted from Vehicle Code section 27156 unless each advertisement contains a legally adequate disclaimer in type size adequate to give reasonable notice of any limitation on the sale or use of the device, apparatus, or mechanism.
(3) No publisher, after receipt of notice from the state board or after otherwise being placed on notice that the advertised part is subject to and has not been exempted from the provisions of Vehicle Code section 27156, shall make or disseminate or cause to be made or disseminated before the public in this state any advertisement for add-on or modified parts subject to the provisions of this article, which have not been exempted from Vehicle Code section 27156, unless such advertisement clearly and accurately states the legal conditions, if any, on sale and use of the parts in California.
(4) The staff of the state board shall provide, upon request, model language which satisfies these requirements.
(c) No person shall advertise, offer for sale, or install a part as a motor vehicle pollution control device or as an approved or certified device, when in fact such part is not a motor vehicle pollution control device or is not approved or certified by the state board.
(d) No person shall advertise, offer for sale, sell, or install an add-on or modified part as a replacement part.
(e) The Executive Officer may exempt add-on and modified parts based on an evaluation conducted in accordance with the "Procedures for Exemption of Add-on and Modified Parts," adopted by the state board on November 4, 1977, as amended June 1, 1990.
(f) Each person engaged in the business of retail sale or installation of an add-on or modified part which has not been exempted from Vehicle Code section 27156 shall maintain records of such activity which indicate date of sale, purchaser name and address, vehicle model and work performed if applicable. Such records shall be open for reasonable inspection by the Executive Officer or his/her representative. All such records shall be maintained for four years from the date of sale or installation.
(g) A violation of any of the prohibitions set forth in this section shall be grounds for the Executive Officer to invoke the provisions of section 2225.
(h)(1) Prior to January 1, 2009, the Executive Officer shall exempt new aftermarket catalytic converters from the prohibitions of California Vehicle Code sections 27156 and 38391 based on an evaluation conducted in accordance with the "California Evaluation Procedures for New Aftermarket Non-Original Equipment Catalytic Converters" as adopted by the state board on August 19, 1988.
(2) On or after January 1, 2009, the Executive Officer shall exempt new aftermarket catalytic converters from the prohibitions of California Vehicle Code sections 27156 and 38391 based on an evaluation conducted in accordance with the "California Evaluation Procedures for New Aftermarket Catalytic Converters" as adopted by the state board on October 25, 2007.
(3) No person shall install, sell, offer for sale or advertise, any new aftermarket catalytic converter in California unless it has been exempted pursuant to the procedures as provided in this subsection.
(4) For the purposes of this regulation, a new aftermarket catalytic converter is a catalytic converter which is constructed of all new materials, is not a replacement part as defined in Title 13, California Code of Regulations, section 1900, and is not an original equipment catalytic converter. A catalytic converter which includes any new material or construction not equivalent to the materials or construction of the original equipment catalytic converter (e.g., an original equipment catalytic converter can with a new non-original equipment substrate) shall also be considered a new aftermarket catalytic converter.
(i)(1) On or after July 1, 2008, or after 30 days from the date of filing of this subsection with the Secretary of State, whichever is later, no person shall install, sell, offer for sale, or advertise any used, recycled, or salvaged catalytic converter in California.
(3) For the purposes of this regulation, a "used catalytic converter" is a catalytic converter which is not a new aftermarket catalytic converter as defined in Subsection (h)(4), or a replacement part as defined in section 1900.
(j) The Executive Officer shall exempt aftermarket critical emission control parts on highway motorcycles from the prohibitions of California Vehicle Code sections 27156 and 38391 based on an evaluation conducted in accordance with the "California Evaluation Procedures for Aftermarket Critical Emission Control Parts on Highway Motorcycles," as adopted on January 22, 2009, which is incorporated by reference herein.

Below is a list of products that are California Compliant for the years specified: