Motor
I ran into the same problem last year when I removed the kitchen window. Screws were rusted and kept shearing off. In actuality , the only thing holding the window to the frame is the screw head. So- if those are sheared off you can actually remove the circular piece holding the window to the wall and then pry the window out. Once the window is out you can access the remaining shaft of the screw from the backside and I think I used a vice grip to do it. Actually, it was one of the forum members from Florida who told me about this---maybe treeclimber, I cant remember. At any rate- I fretted about it the same as you but if you cant get the whole screw out then just cut off the head with a dremel cutting wheel or some similar tool- like that horizontal oscilating tool I mentioned before. One the window is out you can extract whats left of the screw.
post script- just read treeclimbers post
my memory is pretty good. However, I tried replacing with stainless and had issues with them stripping. My local rv guys said he was not surprised and says he uses zinc
Rick