From CA DMV:
Eligibility for on-highway registration
To be eligible for on-highway registration, 1978 year model and newer off-highway vehicles with an engine size of 50 cubic centimeters (cc) or larger must have an emission label affixed to the vehicle indicating certification by the manufacturer for on-highway use.
Exception: 2005 year model or older motorcycles with an engine size of 50 cc or less are exempt from the emission label requirements.
Emission systems cannot be converted to qualify for on-highway registration. For further information on emission standards, contact the California Air Resources Board at 1-800-242-4450.
CA DMV Website: How To Register an Off-Highway Vehicle