I'm looking at building the GP 20 size cub- I'm mostly interested in scale-like flight, nothing crazy. Would an os .15la be a good size for this plane/flying style? (I know, I should put a four stroke on it, but it seems like no one makes a four stroke under .30 anymore). I have a 25la on a slowpoke and it flies pretty quick just on 1/2 throttle- that's why I think a smaller engine would work on a plane of a similar weight. If anyone has any experience with this, let me know how it went.

I bet that it will work. Remember that this plane is a floater and does not need to much engine-power to fly around.
I would buy a 4 stroke .40 size (I have a saito 40 size) and fly it at half throttle with a scale sound. Nice isn't it?

That sounds kinda big... maybe I should ask, and this question could go to anyone with cub experience, do cubs fly fine with the smallest reccommended engine? A small engine would fit better in the cowel, and I think it would make it fly more like a cub, but would running the smaller engine be sufficient for safe takeoffs and emergency power?

Hi!
.40 is too big! A more suitable engine would be an OS FS.26 or .30