Check out Legacycentral.org , there's tons of info there, lots of people convert to GD WRX EJ20's, might be easiest to swap to the EJ22T and 3.9 5MT that came in the BC platforms.
Honestly the easiest would be to just buy a whole new car though. Just see which shells cleaner and swap the appropriate parts !
Just bring $$$$$$$ ! Lol, good luck !