We dont have it too bad here. There are resttrictions, but generally cops wont push too hard, especially on cars older than 1983. Thats when the federal design laws for cars came in. Generally they want you to have the car as it was realeased stock, or as close as possible. Had a cop quetion me once where my catalytic converter was on my Roadrunner, I told him they hadnt been invented yet when the car was sold new.
Engine swaps are harder, but they will generally allow engines that were available from factory, within a couple years of production.
We also have EPA here. Got a letter once from them, had a neighbour complain (was living in an apartment building) about the exhaust fumes. Granted it was smoky, but it was only 1 day, till I got it sorted. He sneakily walked past my garage and took down my plates os he could report me. Seems like a bunch of nancy's with nothing better to do to me.