AFAIK, you'd need to import multiple SP20s in order for the NHASTA (safety place whose name I forget) to perform tests to see if it's safe enough. If you can somehow prove to them that it performs on par with the Protege5 as far as crash tests go, I think you'd then come to the task of proving the engine is up to spec as of the year the car was produced along the lines of emissions standards. I read that one guy on the forum managed to import one in Canada... But he managed to get it there by chopping it into three pieces- they wouldn't let him drive it on the street or something like that.
Hopefully someone can chime in and correct me as I'm probably wrong in some point of my advice. I looked at what it would take to import a 1992 Escort Cosworth. On top of fees to import it, buying the car, and talking to the gov't, it's probably not worth it IMO.