after years of sneaking through the smog checks with my '77 911 with a later engine conversion, I got caught and had to make an appointment with BAR referree. The engine serial# was actually a '78 turbo, although it wasn't a turbo anymore. On top of that, I put a '83 injection on it so I wouldn't have to mess with air pump. Further complicating matters, it was a 49 state car and the "noncatalyst" sticker was blown off the door jamb during a re-spray. So, yeah, I was glad they would be testing the car and not my blood pressure. Result:
they accepted my word as to the year of the engine and tested accordingly. It passed and they put a sticker on the jamb so techs at future smog checks will know that the engine swap was approved and what MY to base their inspections upon.
basically newer engines are allowed, but all emissions equipment associated with the car that engine came from have to be on your car, and must pass the specs of the newer engine. good luck