RE: sea plane for saito fg-36
We can't answer that question without knowing what kind of flying you like to do. Trainer, scale flight, mild aerobatics, pattern, 3D, racing?
The FG 36 is roughly equivilent to a 1.20 two stroke or 1.50/1.80 four stroke glow and there are plenty of choices for those engines in each of the catagories above.
Pick one you like, but make sure to get enough wing area to carry the extra weight.
Edit: You can put almost any airplane on floats, so don't limit yourself to the usual sea/float plane designs.