You could use a depth gauge with a small diameter probe on the end. With the piston up measure to the side of the piston, then to the far side of the bore with the piston down.
The baffle would make things a bit tricky, but with a bit of fiddling (small bend in the rod to clear it) it should be possible.
The motor would need to be fixtured to something as well, but that would be fairly simple.
You don't need to be too accurate, .8" versus something like .73" should be easy to measure.