Actually there are quite a few non comforming cars made legal in the US including 959s, Mclaren F1 , as well as other rare cars. Just costed a lot of money for the people that brought them in. To get a silvia legal that way, it would have to go the same as the skyline though and probably have to be converted to LHD to make things easier. But this however is not an ecomically viable solution for the silvia as you would end up with a $40-50k silvia (S15 because s13s and s14s are kinda pointless when you can just make one) which no one would buy. There are however quasi legal ways to bring in almost any car you want. Look up Parts for Assembly vehicle...ie kit car.........

Also for motorex to get this done the crash tested a lot of cars becuase they had to do it for ever model of every generation and since they sell the GTS and GTR, thats 6 deifferent variations times several cars each. I dont know how the RIs legalize the exotics like the Mclaren but i know they had to put on different bumpers and had to take out the two side seats, but i doubt the crash tested any here