The engine looks exactly like one I have. It appears to be either a U.S. Engines 35cc or a 41cc. They look almost the same from the outside, mine is on the front end of a giant Stinger. If it is a Stinger, it appears to have a modified canopy and vertical stabilizer. One way to tell if it is a Stinger (at least an kit built one) is to look in the fuselage. If the fuse is all constructed from light ply, chances are that it is a Stinger...

Nice find for $20.00!