How can I fix the stalling of an RC plane after the addition of floats?
Your model is based on the FX-803 Piper Cub model shown in this video. It's made from expanded polystyrene wings and fuselage, with a two channel radio that controls thrust from two motors mounted below the wing, with overall thrust controlling pitch and differential thrust controlling yaw. There are no moving control surfaces, and the propeller on the nose is a cosmetic windmilling attachment.

Your extra wing, lashed on with elastic bands, may or may not produce lift, but adds additional weight and drag below the thrust line, inducing a significant nose down moment. Your carved foam floats add additional drag even further from the thrust line exacerbating the nose-down problem, and further increasing the weight.

The original model was built and trimmed to give flying characteristics that could be controlled by simple thrust adjustments. There is no moving surface with which to counter the added drag, and even if there were, it is unlikely it would have sufficient authority to overcome the problem. 

The modifications may also have moved the centre of gravity significantly, possibly beyond the point where it is fixable.

The model shown in the video has sprightly performance on limited power. The additional weight and drag you have added could easily overwhelm the available power. 

You could try adding a fixed elevator to provide significant down force at the tail, but this will further compound the drag issue, and could also affect centre of gravity.

Realistically, I doubt you will be able to get this to fly. If you do, please come back and post a video!