soos- you might still be able to measure it, even though it's on the bike already. the hard part will be sealing the rings, but i'd try the following: squirt a measured amount of heavy-ish oil into the spark plug hole at BDC (like if you were doing a leakdown test and trying to seal the rings to figure out if a compression loss is in the valve train). you can probably find something to run down the spark plug hold and feel for BDC. then fill the combustion chamber with a carefully measured amount of food-colored isoprobyl alcohol. mine measured 185/17 (BDC/TDC) with 63mm pistons. i'd start filling really slow around 160cc's to make sure you don't overflow and lose your measurement. fill fast though: the oil might not seal the rings well. the 650 spark plug hole sits relatively on top of the cylinder, so you should be able to fill the chamber to the top.
you can use an ear syringe with some tubing on the end to suck the alcohol back out of the cylinder. id also either give some time in between the two measurements to make sure the alcohol evaporates completely, run it in between, or use two different cylinders for the measurements since you can take it apart and clean it up.
