Hey Brisco welcome to the forum.

OK first off I am not a stranger to float planes so I think I can offer some useful information and I have flown in the past some forty sized Decathlon on floats that I was helping a fellow with. Can't remember if it was GP, CMP or the Pheonix from viet nam.

Anyway the weight as you described is completely within the ballpark for that airplane and yes float planes are heavier but even with the extra weight many will fly better as the floats themselves act like stabilizers.

One item I would like to address is the Step/CG relationship and for that size airplane is a little much at 3/4inch aft. This is a item that may make it harder to get up on the step and that is not what you will want for an airplane that may be underpowered but more on that in a bit. For that airplane I would prefer to have the CG anywhere from directly at the step or up to around 1/4 inch aft of the step. Correction: what I intended to say is The Step up to a 1/4 inch aft of the CG.

On the power thing, The trouble with the little fourstroke you mentioned is first its not a powerhouse and it is grossly underpowered even on wheels. I don,t care what factory recs say and often some of them are just hogwash. Now the airplane I mentioned that fellow had put a small fourstroke of some sort on it and I flew it, and as expected it was a dog and underpowered. The real problem is an underpowered float plane seldom lives very long especially when the fellows are new float flyers.

Now the fellow with that Decathlon did as I suggested and that was to get over the fourstoke phobia and use an OS 55AX or even the .65. He did just that and the result was an entirely different airplane that be became a joy to fly. The whole gotta have a fourstroke thing is overrated and many airplane types work better with the two stroke.

If you must have the fourstroke my minimum would be .70.

Here are couple of pics of that ship I was talking about:


The last two photos is what I did with my Decathalon, turned it into a Champion Lancer or as some would say the Flying Turd: