Dude I am generally nice to new guys, but there is a post talking about this on the first page of the SR20 forum about 10 posts below yours!
No the SR20DET is not legal in the US, it never has been, never will be, there is nothing you can do about that, you can just get your car inspected / pass emissions and live happily with an illegal engine. Unless of course you wanted to drop $40k or whatever to homologize a 180sx or Silvia and bring it over. But then I would have to fit you for the roman helmet!