Putting aside the enforcement aspects of this (which give rise to a whole lot of practical questions), the proposed regulation seems pretty clear. If you alter the emissions equipment on an EPA-certified motor - even if it's going in a car that will be for track-only use, you're in violation of the Clean Air Act. And since the EPA is characterizing this as a "clarification" of existing law, it is not prospective only and would be applicable to lots of track and autocross cars that aren't licensed for the street. If you have a motor that was built from scratch by Roush or folks of that ilk, you might be ok. All in all, troubling to say the least, but again open to questions of how EPA would enforce the regulation.