Importers and Exporters

Asian Trading Online has a summary of US laws.

NHTSA Importation and Certification FAQ lays out the law clearly.

If a motorcycle or motor driven cycle is capable of a top speed above 20 miles per hour and is equipped with components (such as lights, mirrors, and turn signals) that are needed for on-road use, NHTSA will regard it as having been primarily manufactured for such purposes. Motorcycles and motor-driven cycles with these capabilities and equipment cannot be lawfully imported into the U.S. unless they were originally manufactured to comply with all applicable FMVSS and bear a label certifying such compliance that is permanently affixed by the original manufacturer.