There are 4 basic engines...
EJ205 - 2.0 litre wrx
EJ207 - 2.0 litre STi
EJ255 - 2.5 litre wrx
EJ257 - 2.5 litre STi

Any of them will fit in your engine bay.
As everyone keeps saying, buy a 207 short block.

There are a number of other engine variants, but these are the only ones you need think about.
If you want to learn more... http://en.wikipedia.org/wiki/Subaru_EJ_engine

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