Jeff, I looked into this a bit today, visited a guy who builds KZ900s/1000s and and compared a 650 tensioner to a 900/1000 tensioner, here's what I found:
- The shaft that holds the tensioner bracket in place (rear cam chain idler gear shaft) is the same on both tensioners (also same partno: 12052-005).
- The upper holes in the bracket are the same size. The brackets are also equally long from pivot point to wheel center meaning that the backplate where the tensioner plunger contacts the bracket is positioned right.
- The only issue I can see is that when using the 900/1000 tensioner on a 650 engine, the tensioner plunger needs to be a bit longer than previously assumed - apart from the 900/1000 wheel being smaller the bracket is equally smaller front-to-back. When aligning the front of the wheels (the side where they contact the chain) there is a 10 mm difference at the backplate as shown in the pics below. This shouldn't present a problem with a manual adjuster though, if so it shouldn't be very hard to fab a manual adjuster with a longer bolt.
Btw, while I was there the guy showed me a japanese catalogue with an interesting tensioner mod: rather than using a rubber wheel they installed a cam chain idler gear in the bracket - this can't be done with the 900/1000 bracket, the gear diameter is too large to fit in the bracket, but the idler gear is the exact same size as the 650 rubber roller and fits in the 650 bracket, might be worth looking into.
Anyway, looks like it shouldn't be a problem using a 900/1000 tensioner in a 650 engine, I'd be willing to give it a try myself.