I have done----Silver solder.---Alumium brazing.---Brazing with bronze.-Aluminum TIG welding.----Stainless welding.----Stick welding of course.-----I do not think welding this engine block is a good idea
Does not look like there a place for a center board or swing keel.----Cabin sole look like fixed keel to me.----And I have sailed on many different boats.----From a Laser to 62 foot ( 55,000 lb ) go anywhere in the world type of boat.
I would say the voltage should rise right away.----It does on other motor brands.----Have you considered getting a pair of different motors.----Seems there are a lot of issues.