Wouldn't worry about it as a cub has a bunch of lift. Being an ARF there isn't a lot you can do about it other then strip the covering and start lightening. Last one I built was the Goldberg Cub and it was heavy at 8 pounds with an OS .91fs. As a 40 size plane I thought it was heavy but the wing is built like a tank and it has a lot of ply in the build. In flight it is perfect though. The .91 was the only engine the fellow I built it for had but a nice .46 two stroke would do just as well. He added floats and figured he would need the extra power anyway, I don't think so but you use what you have.