" How about a small Yamaha Motorcycle engine!!!! Capt,n"
More better, Yamaha KT100 Kart engine. Almost 25yrs ago I had a friend with a Wizzard weight-shift ultralight on floats. Powered by a KT100. I'd bet an RC plane on one of these would kick butt.
I think they make 20hp. . There are zillions of them in the Kart world, I don't know how cheap but worth a look.
MIKE
EDIT: [link=http://www.radne.se/Store/Raket/]Raket Engines[/link] They make a 60, 85 and 120. Popular in Europe. Used in ultralights too. Bone up on your Swedish.