isnt there the idea of a car being 'street legal?' we already have an incredible amount of regulation on how you are able to modify your car. You shouldnt be able to do anything you want to your car and expect to use it on public roads.
In the US as long as you don't touch the emissions system you'd be hard pressed to run afoul of anything as long as you're not taking some specific look and chasing it to the extremes (super tall trucks, cars with tires sticking way out, absurd headlights, etc).
Very few places have restrictions beyond your basic safety equipment (seat belt, airbags), turn signals, reflectors, being installed and working properly.
Smog isn't a requirement in a vast majority of US counties, and even in the ones where it is required, finding a shop that will pass you for it even without a catalytic converter is fairly trivial. Further, I've never heard of anyone getting ticketed outside of California for having smog noncompliance.
You're describing fraud, which is an issue orthogonal from standards compliance.
If you're worried about people surreptitiously breaking the law then it doesn't matter what the law is because those people are breaking it regardless.
