Now, from the Jalopnik link that gsmith2424 linked, it's interesting that the EPA is outright saying that enforcement priority will be very low for actual race cars.
Here's what's going to happen.
Shops that sell parts for actual race cars that are actually proven to be decommissioned for the street are at some risk of prosecution, but generally rather low.
Race teams will have almost zero risk of prosecution, even if they fabricate their own parts - the EPA very rarely goes after individuals, leaving that to the states.
"Race" shops that sell parts for "race" cars that are actually being used on the street? Hammer will come down on them, and this means that the EPA doesn't have to listen to excuses that the parts are only for competition, because they're still illegal unless they're going on a car that was never certified for the road.
That said, making things illegal and then saying you won't enforce them is a dangerous situation for obvious reasons.
There is a way that the EPA could go about this that could accomplish their goal without hurting actual motorsport, though, but it'd require a significantly higher regulatory workload for them, for states, and for manufacturers of motorsport parts. The EPA could implement a production race car conversion registry, storing VINs of cars that a race team wishes to decertify from road use. Once a VIN is in that list, states must void any active registration on that vehicle, and refuse to ever accept registration for that vehicle again (effectively, RACE CAR would be a title brand that would prevent registration). And, once a VIN is on that list, emissions tampering and defeat devices for competition purposes become legal, but selling services for tampering must include the VIN of the vehicle worked on, as well as serial numbers for any defeat devices. So, if the EPA suspects a shop of selling parts for street cars, they can cross-reference the VINs and serial numbers.
That's a hellaciously complex system to implement, though, and it still opens some loopholes (the "but I sold it to my customer for their race car, not my fault they took it off and put it on their road car" loophole), and may encourage VIN fraud.
