The 650's not a bad bike. As said, parts can be swapped from the 650 back to the 550 (probably 500, since the 500 and the 500 are more similar). Paulages (here and on the SOHC4) has built an engine comprised of 550 and 650 parts w/750 sleeves and some machine work and I think the displacement is 718cc or close to it. It will scream. I believe he used the 550 case with most everything else being 650. Now, the 650 doesn't have a kick starter, uses a hypo chain from the crank to the trans and, I am not sure but, I think
the trans gearing might be different. I am told that the 650s have some differences in the different models also. like the 79 and 80 are grouped together while the 81 and 82 had some differences like carbs and I'm not sure what else. I'd really have to check. I SO however have a buddy over on the SOHC4, Terry (uses the username of cb650, if that tells you anything, that knows a bit about the bikes and the pros and cons of the different year models.