My thoughts would be to not cut the existing sponson inside if you can avoid it. You already have the spar and canard mounted to it so that would save some work. If the deck and bottom aren't damaged where they are attached to the sponson inside, I would leave some of that as well. That would give you an area to attach a doubler to the inside that the replacement piece can be fitted and glued to. After that is done, I'd lay a thin layer of glass(two if you prefer) along the joint to give the repair more strength, fair it out with whatever you like and paint.
I'm sure others will have a different approach to how to repair that sponson. Personally, I prefer wood boats as, for me anyway, they are easier to repair