couple things to remeber
1) subaru engines are wider
2) they arent a direct bolt in (cutting required as far as i know)
3) they are watercooled (have to find a place to mount the radiator, and run lines.
4) they are watercooled
5) they use water instead of air
it would be cool to see a subaru engine in a bug, but for the time money and trouble why not build a performance vw engine, thats bolt in
-dan