Cub on floats
I would get the GP 40 size floats. Take you about a week to build depending..But are well designed and will have instructions on mounting them to your cub...
Any 50 sized 4-stroke will be fine and add to a more realistic flight. Most of my floaters have 4-C on them and have never had a problem with them.. If you get water in them, just flush out the carb with fuel..
Any cub on floats is an excellent choice. I learned to fly RC with a cub on floats..What you have is a scale rc trainer....good luck you'll love flying off water....