World Models Spitfire:
http://www.airborne-models.com/html/...p?ProductID=55

This plane would fly very well with a 50cc engine.

Kangke Waco Bipe:
http://www2.towerhobbies.com/cgi-bin...?&I=LXHRD0&P=0

Lanier Stinger ARF
http://www2.towerhobbies.com/cgi-bin...?&I=LXLDE4&P=0

I know that you asked for an ARF--but Top Flite still makes some pretty cool kits for old warbirds in GS size. You'll just have to build it. The F4U Corsair and the P-47 Jug come to mind.

I thought that H9 95" Giant Cessna would heva been a good candidate for a 40-50cc gasser. Probably overpowered--but thats okay. It's currently out of production untill late spring 2006.