I'd say just drop the 2.5L sti engine in it. Its gonna be way too much trouble to do anything else and I think you'll be very happy with the STi engine. Its already together and working. You also can get the whole drivetrain/tranny setup from an sti for 2-3k. You'll want that trany like everyone else is saying since it's practically bullet proof for dragging/racing. Just make sure to use good fluid .