Here's my view. Does the 636 use a cassette-type trans? If not, you have to split the cases. Not an easy job. Plus, who knows what else is wrong. And after all your work, you have a basic 636.
If I was going do do a lot of work on a bike, I would get a good running ZX-11. Then I'd put on aftermarket suspension, brakes, windscreen, and seat. At least then I'd have something unique. A comfortable rocketship that handles and stops.