I'd be getting another F20 in there, or a K20 perhaps. I like the idea of a 6cyl Turbo from something else if you're after more power, but it would end up pretty expensive to do well!

One option which would be cheap (looking just at engine cost) would be an SR20 DET from a Nissan 200SX, plenty of aftermarket potential there too, it would just be getting the thing mounted and wired in that makes it more painful!