I have heard that US engines of a given year are far more compliant than a similiar year here.
Say a 2001 Mustang engine meets our 2004 here.
Check with the registration people as they will ultimately be saying yes or no.
Additionally, there would be emission specs with the engine you purchase. A 2004 engine will/should have emission specs.
I cannot see a problem as I can recal Mike Moore registering his Extreme Roadster with a new LS1 motor (import)
Just my $.02 worth..
Bernie