I have a 1/4 scale cub with a full wing on floats. I have a 26cc RCCR and it has more than ample power. Takes off water on 1/2 throttle and flies on 1/4 throttle. I'm not endorsing RCCR as there are many good engines out there. I've just had good luck with mine.
I've never weighed miy Cub but I don't think its over 15 lbs. One thing for sure, you will be happy with gas over glow.
Hope this was of some help.
K.O.