The World Models has a wonderful 1.20 sized Ultimate Biplane that would easily take a 20cc gasser. I had one that I flew a lot for two years before passing it on to another modeler. I had a 2-stroke 1.20 glow engine in it, but I have no doubt that it would handle a 20cc gas engine.

Check it out [link=http://www.airborne-models.com/html/productdetails.asp?ProductID=85]Here![/link]