That would make a nice size floatplane. Take a look at Saito, they make both a .30 and a .40 4-stroke, at least they did. I think the .30 is discontinued, but there should be plenty still in the hobby shops; my LHS in town has one. I agree with Nony, though, and would opt for the larger .40. It's still a small engine and should fit okay. Whether OS or Saito, that's a Ford-Chevy choice. All my engines are Saitos, I'll stick with them.

The Great Planes floats are easy to build. There isn't much difference in how all three sizes build; as a matter of fact, the instruction book is the same for all of them. I fiberglassed and painted mine; I think it's the most durable and waterproof way, but you can MonoKote them also, just take especial care to seal the seams.