You might want to take a look at the 25cc Homelite conversion engine produced by GCBM RC Models Inc. (they run that ad in Model Aviation for "25cc Gas Engines for $210"). I've got one on my Sopwith Pup & I like it. Easy to start & solid performance. It is heavy, but I'd have to add ballast if I went with a more expensive lightweight engine anyway. The previous owner of this Pup had a G-23 which is very light, but had to add ballast. I don't know about the nose moment on the Cub, but if its short, you might find the 25cc Homelite conversion engine to be a good fit.

best regards.