Squish can be measured with a piece of thin electrical(non flux) solder and some calipers, its just more annoying on a single piece jug since you'd have to pull the whole jug, tape it to the piston, then reassemble to rest squish before removing it again. Just make sure to tape it along the wrist pin and not across it since the piston will rock in the bore some and mess up your results.
With a mill you can just cut the stock head off, mill it square, then use any aftermarket head to make it much more mod friendly.