Been researching motors for my build… especially electric motors. I would really like to go down this path, somehow.
I may wind up with a source for a very cheap or free outboard lower end unit, and several people have converted these into electric outboards. Fair amount of information out there.
The biggest issue that people seem to have is making a coupler to go from the (usually splined) drive shaft of the lower end unit to the keyed shaft on the electric motor. Well, finally found what looks to be a reasonable answer (that I see many in the electric vehicle world have stumbled upon, but I hadn’t seen it much in the electric boating world.
It’s the Lovejoy style coupler. Its a piece that fits on each shaft and meets with three offset "teeth" in the middle with a filler of rubber/plastic/bronze (depending on needs) material to take the slack. Lots of combinations possible.
Lovejoy has lots of details on their web site, with PDF catalogs of all manner of parts you might need.