...Soooooo, my question is, is there a way to check the displacement without taking the head off? I understand the principle of cc'ing a motor. The stroke would be known, so by knowing the amount of liquid used, the bore could be calculated. EXCEPT, I would need to know something about the volume of the combustion chamber, piston top shape, etc.
Any ideas?
You could try to flashlight into the sparkplug hole, and see if the pistons show a small flat ridge that Jay760 mentioned in this other thread.
http://www.xs650.com/threads/how-to-tell-if-i-have-a-750-kit.42213/
It's only about 0.100" wide.