650/750 starter compatability

Will a KZ650 starter bolt up to a KZ750?

I would first ask if your 750 is a twin or a four. If it is a twin, I do not think a starter from a 650 will bolt on. If it is a four then yes it is my understanding that it will bolt up. Keep in mind that some of the starters stock on the 650s are 2 brush starters and some are 4 brush starters.

yes...it's a 4 cylinder.  I have a spare 650 engine, condition unknown. I suspect my starter isn't up to snuff so I was thinking of swaping out the starter from the 650

OK, so having read your other post re headlight issue when cranking the starter, it could be the starter or the battery that is causing the issue. I have found this Mike Nixon post to be a great home made tool to check the starter and the charging system. Keep in mind that the starter is by far the largest draw on the entire electrical system on the bike.
 

..... I suspect my starter isn't up to snuff so I was thinking of swaping out the starter from the 650

load test the battery. it's battery season. from your other topic that noted 8 VDC when trying to start the motorcycle it sounds like the battery needs replacing.