I live along the southern US border near Mexico. I see many Mexican-plated cars and motorcycles/scooters (Itallika seems to be the primary Chinese derived motorcycles/scooters in Mexico-not sure manufactures for them ) driving on the US side of the border--cars that don't meet the US DOT/EPA rules, such as the last years of the VW Bug. The are allowed in as vistors or working commuters. Keep in mind they are from Mexico, and many US plated cars drive south into Mexico, too, so I am assuming there is some sort of agreement that allows this.

Coming with Chinese-plated bikes would probably be allowed, although I am not sure on the process--would they be allowed I could try asking one of the many US Customs Agents that live and work here, although they might be clueless. I think, but don't hold me to it, that it would be no different than Mexicans, Canadians or others. I have seen British-plated cars on the road in California, for instance, and a German-plated car in Arizona.

The two paragraphs don't seem to contradict each other, because the second one is talking about racing vehicles for racing purposes, while the top one is talking about personal vehicles for up to a year. It does state that the owner and vehicle arrive together. Worse comes to worse, the bikes have to be exported if they don't meet EPA emissions, but I know that some of these bikes have come into the states, so might even be able to be sold here if the riders don't want to send them back!