I did do some research previously. The legislation is Trump era. The enforcement is Biden era. And the legislation does appear to outlaw modifying street cars into racecars. It just hasn't been enforced. The RPM act is the attempt to protect the automotive modification aftermarket and racing https://www.sema.org/epa-news , but it's been languishing and hasn't passed. When I mentioned boat and airplanes, I was talking private not commercial and was think hobbyist not OE. I don't think the EPA has gone after boat or airplane tuners in the name of the Clean Air Act like it has in the Diesel/car aftermarket.If you did your research, you would see this started in the Trump era EPA - Just the current administration is allowing it to continue unabated.
Also, boats in port have to follow current EPA rules, UNLESS they receive a waiver. I had to work on a cargo ship that had it's EGR blower "waivered" as the equipment was "blown up" by a huge electrical discharge of some sort. Ship was in port for three days, and left to head to next port for the repairs to be completed.