I just picked a 1981 (according to the data plate) yamaha xj and unfortunately its missing the body panels, head, and all the subsequent parts; and i have no idea what size the bike is so i dont know what to buy parts for. How can i positively ID this bike?

Normally at the base of the cylinder there will be a little block with cc's on it. Round that up to bike size.(because it is probably 649cc,549cc,etc.)

Ok now that i found it, (i thought it would be on the side somewhere but it turns out it was on the front), can i put 750cc cylinders on a 650?

I checked bikebandit.com and the cylinder base gaskets are the same so i hope i am not way off on my assumption when i pick up a top end off of a 750 instead of a 650.