RE: piper j-3 cub
I have the GP Piper Cub J3 40 kit. We maidened it last week with an OS 40FX. Its a great plane, files very steadily and the engine is way too much for it. Its pretty easy to build too.
I think you should get the 40 or the 60 just because then you will have an engine for your next plane after it.