You'll be better off glassing in a new stringer. Make a jig that will place it exactly where the existing one is and get to work. It will be stronger this way. Right under the bow is probably the spot where the stringers go through the most torture, so you want to make sure you have one pice there.
Ill need more detail on your motor mount idea. Its fine for the stock mounts to bolt up to the stringers... You could go with a rail kit, but I dont think they are any stronger, just less stress on the stringer.