Going into the lower end is a lot of work and expensive when you start buying all the bearings.

If you absolutly have to, and you can not find a better motor somewhere else, then one big thing to check out is the shift drum and shift fork assembly. Those are notorious for going out on 750s and there is no point in going to all the trouble of working in the lower end and not checking that out.

If you don't have a book of specs yet, you need one now or you are wasting your time.