I'd think a 4-stroke 80 would be overkill.

A friend just completed Sig's 1/5 Cub and flew it Thursday on wheels with a Saito 50. Took off with great authority and cruises realistically at 1/2 throttle. Plenty of power, I think, for floats. The Saito 50, BTW, fits inverted completely inside the cowl.

I fly a Sig 1/6 Cub on floats with an OS FS 26. My Cub is light and I like realistic flight, but with just the 26, it takes off with floats very nicely and cruises at 1/2 power.

Another friend has a Goldberg Cub (sorta in between Sig's 1/6 & 1/5 in size, but heavy) with an OS FS 61, and we call it "the FJ3" Interceptor.

-Dave