If you want STOL abilities go to a oversized 2 stroke. I do this 2X the rated C. I. size on every plane I have.

Build the plane. Get all your rc in place & woking. Mount the fuel tank. Install the floats. Mark COG to balance point on the wing with tape. Put a piece of Balsa under each C O G point on each wing...To prevent digging dents into the wing when you do the next step.
Start adding weight on the engine mounts. Keep adding weight till the plane balances. That is the weight of the engine your plane needs to fly in ANY wind and it WILL STOL.

You should be able to hold the plane vertically and have it fly straight up & out of sight.
The plane will normally fly on 1/8 to 1/3 of throttle & sound perfect.
I Have found only 1 slight problem. Idle speed can be a little to high. 1 plane. a Spitfire must be flown to 6" of the ground and I stop the engine to slow down for a tail wheel landing.

Overpowered plane is a joy to fly.