Looks like he's no Monkeyclaw type, and he sports a nice group there.
If you're near by, I'd certainly check him out for your 750 needs. Bone up on pricing and ask ask ask questions before so you don't fall for any shenanigans.
Warranties expressed or implied? Year motor...miles if known...recent rebuild...etc.
Hopefully the guys here will toss some numbers out regarding pricing, as I have no clue what the value is for a rebuilt motor, or just a flat out ballpark price for a "known good motor".
This is always tough when you have no known references...hey...maybe ask for two references, and actually call them if he's the garage mechanic who flips bikes, he should have no problem with that request.
He does have some nice looking stuff there.