While the 4ag silver/black top would be a great choice in terms of engine swap it is illegal! Unless it is a track whore don't get caught with an old engine in a new car.
It would be great to have a Formula Atlantic engine in this thing OMG. 11k rpm 4ag 240hp
pretty sure its the same as in Initial D. Itb's and all.
STI swaps will be very popular i think, since it will just bolt right in. Now turbo clearance etc... will/might be an issue but they seem to figure things out pretty quick in the tuner world.
WRX swaps as well (the new 265 hp variant). Should be cheap at a salvage yard etc.
Until we see the engine bay there is no telling what engine would fit.