As
@Dilldog posted, Federal motor standards are fairly strict, the state can decide whether or not to ignore them. This is why you can drive something in one state without issue but have your vehicle impounded in the next. Again, just because you see them doesn't mean its legal.
In California, regardless of "25 year import law" , any vehicle made after "1976", you must meet emission standards. In CA we just don't test before 76. By federal standards, any vehicle that came with an emission device, regardless of year, cannot have the device removed and regardless of year (or country of origin), the vehicle must meet those standards. If the motor was never made for America, it wont smog.
I have been hearing some things about Colorado recently. I'm beginning to believe, they just reset the bar on vehicle emission requirements. Things we allow here in CA wont pass there.
The other issue is most states have a law against right hand drive vehicles. It doesn't matter if you see right hand drive vehicles on the street, it's based on, "If the officer decides" to enforce it. The import law just allows you to bring it in. Its up to you to know what your state laws are.
You are also not allowed to modify suspension, or drive line outside of what the factory equipped it with. When the vehicle was produced, it was certified safe for America by the manufacture, with these items. This includes changing tires and rims outside what was offered by the manufacture. Also except for a few specialty vehicles, no right hand drive vehicles were ever made for America.
Allowing the states to make their own version of the laws now create grey areas and confusion.
A good lawyer can make your life miserable.
Realistically, every vehicle I have owned violates most federal and state laws for chassis. In my entire life I was only cited for my mods once. For this reason, my mods are always subtle. In my local mountains, anything "Jeep" with a lift is cited if they are caught. They really enforce the height and tire laws there.