If you are trying to find a 4 cycle to fit it, there are a few available. Magnum makes a .54, OS makes a .40, and a .52 and thundertiger makes a .54. I would think any of these would work fine, and the weight of a slightly bigger engine (than .45) shouldn't be an issue with a cub's wing loading. I would be hesitant to use more than a .40 sized 2 cycle on this plane. The Great Planes model in the previous post is a little bigger, (roughly 200 more sq in of wing) That higher performance 46 on yours would be overpowering.

Hope this helps,

Dan