The older a car is the easier. There are literally hundreds of S30 Fairlady Z's running around the US that are 100% federally legal, mostly due to the "Twenty-Five Year Rule" which allows cars older than twenty-five years to skip some of the hoops you usually have to go through.

I do find it funny though that Japanese safety requirements are always called "Less strict" than ours, yet they have a much lower accident injury and accident death ratio than we do versus the number of accidents they have. I'm sure a few people will pipe up with "They drive slower" but that's not quite true, the electronic governors stop you at 180km/h, 160km/h is around 100mph, more than fast enough to get killed.

Anyways, federally legal is a bit of a pain too because of the amount of money that gets paid to the government in taxes and fees for it too, it does have one upshot: No matter where you move, your car's legit.