I had a '79 4 cyl. Turbo and it was a hot-sh#t little car for back then. They also were in the '79 Pace cars.
I know you can start looking for '79 Mustang engines, but I don't know how long they lasted for production.
If I had my choice, I'd stick a 5.0 in it instead of a turbo 4 cyl. Turbos seem to have a lot of problems attached to them.