California Air Resources Board

Replica motorcycles are new vehicles that duplicate the look or technology of vintage motorcycles. They are legal for registration as long as their manufacturer has certified them to meet the requirements of their model year of sale. This means that a motorcycle that resembles a 1955 model, but is sold in 2019 must meet 2019 or newer new motorcycle emission standards. You can determine compliance with emission standards by checking its "Vehicle Emission Control Information" label located on a clearly visible portion of its frame. You can also verify the accuracy of this information by comparing it to certification documentation on ARBs website.

Please note that all new motorcycles must also comply with all applicable US Department of Transportation regulations in order to be legal for registration. In addition, compliance by the manufacturers of these vehicles with California emission standards does not eliminate any design, intellectual property, or trademark rights of the original manufacturer, or prevent them from taking actions to enforce or protect these rights.
 

IMPORTANT POINTS:
All manufactured vehicles, even partially assembled ones must meet emission standards for their model year.
California and USEPA emissions labeling is required on all motorcycles starting in the 2006 model year.
California and USEPA emissions labeling is required on all motorcycles greater than 50cc since 1980.
Safety labeling is required on all on-road motorcycles for California.