I think most .46 sized engines would work pretty well in this plane, however, i haven't heard too many good things about Tower engines. The only Tower engine i've ever seen personally seemed to have an air leak around the carb or something because it didn't run worth a hoot either.

If you already have the engine, and it works allright, then it'll probably work just fine on the Extra. If you don't already have it, and you're looking for something cheap get a Thunder Tiger or even a Magnum. Both are very inexpensive motors that are good performers, especially the TT.