Actually, if you want to spend the money then you can do it. The parts would be more than a new motor but it can be done in some instances. I have a kit to change a 24v motorguide into a 10lb thrust digital 36v that will be installed if my trolling motor ever goes south.
A 40 lb trolling motor is fine for fair weather fishing. I have a 101 but 90 percent of the time I am only using 15 or 20 lbs of thrust. If you care to fish in heavier winds or waves then I would look more into the 56lb thrust motors.