check to see if the heads have the deck height 'built in' some do, then you can almost go level with the top of the cylinder, allowing 0.15 -0.2 mm for piston rock....
if you can be bothered make or buy a cc kit so you know what the volume of the chamber in the head is - then you can plug the numbers into the various engine calcs and determine your CR...
think std heads are around the 50cc mark??