As a practical matter, a lot of it is unenforceable. It has been and still is illegal to put loud pipes on motorcycles, as an example, because noise is an emission. Rejetting carburetors is equally illegal, as is removing the plug that keeps you from adjusting the idle jet.
The problem with race vehicles is that many races are televised, and the TV video will be compelling evidence. Black smoke from diesel drag racing pickups, pictures showing aftermarket exhausts on race bikes, audio of clearly loud pipes, and so on might find its way into EPA enforcement actions.
Personally, I think the EPA is overreaching, a problem common to the present administration (and many of its predecessors). I think, just as with the impending regulations about CO2 emissions (stayed by the courts, for the time being), the whole thing may end up in the Supreme Court.