Stick with the A-4. They're great engines, and inexpensive to fix/overhaul. Take the money you'll be saving and stick it elsewhere in the boat. I have one, and it always gets me home. I'm always having to do this and that to it, but in many important respects it's better than a deisel. I can't imagine an electric motor, with its massive battery bank, is going to be lighter than your A-4.