Aluminum starts to form a surface oxide seconds after it is made. That suface oxide is what helps it keep from further corrosion. You say it looks extremely clean, so I don’t think it really is an electrolysis problem. There is no anode. Those were not in use way back when.
Personally, I think for experience, you should concentrate on making it a perfect running motor for now. If you desire a new-looking motor, that can come later, as step-two. Us old-timers around here appreciate a nice original motor just as much as a restored one. Others over-restore them. Takes all kinds of nuts to hold the world together.
