Anybody know a way to calculate the compression on a cylinder where the head can't be removed?

Can the engine be positioned so that the plug thread is the highest point?
That + burette = solved.

Maybe, it's a Continental opposed airplane engine.

What year and size, both C.I. and H.P., is it? Is it N/A or turbo charged?
Most Lycoming N/A air cooled opposed aircraft engines are below 8.5 to 1 compression ratio, I'm not sure about all of them and or motors like yours.
Those are low RPM motors, usually below 2700 RPM Longer stroke than bore size also, some are rated for continuous full power and some are limited to 5 minutes or less at full power
Bottom line is most of them are not precise engines with tight tolerances on the rotating parts like the turbine motors are

If the rings don't leak, but of course that would mean lower compression regardless of what is measured

Let me try to cover the specks. It is a C-85, which the cranks are hard to find, so they have come up with this conversion, where you put a O-200 crank and rods in the C-85. The O-200 has a longer stroke. The compression on a C-85 is 6.3 to 1, so I was thinking with the O-200 crank and rods longer stroke there should be an increase in compression, but nobody has ever ran the numbers. I was wondering, so the power increases cold be figured out.
The C-85 is a 188 ci, while the O-200 is 200 ci.
C-85 stands for 85 HP. O-200 is 100 HP

I would look (measure) at the pistons wrist pin location and how much differences there are in the rod length and then see if it is possible to figure out how much higher in the cylinders the pistons your going to use will be and se if that can help you figure how much more compression you will end up with.
Are you wanting to use auto gas in the motor?
I owned and flew a Piper Comanche 260 HP for 15 yrs and always ran 100 LL in it even though it was 8.5 to 1 compression ratio, I'm sure it would have ran fine on 86 or 88 octane autogas
Stock ignition timing called for either 26 or 28 BTDC on both mags, me thinks I may have set mine at 29 or 30 BTDC
I lived in SO CA and flew out of L26 which was 3100 FT on the low end of runway 23 and a little over 3250 Ft on the displaced threshold on runway 50, I always did a full throttle run up and adjusted the mixture to between 25F to 50F rich of peak before take off

That might be possible. This is for a J-3 Cub

Math isn't my strong suit, but if it is safe to assume the piston height in the cylinder is the same with both rotating assemblies, you should be able to calculate combustion chamber volume above the piston at TDC based on the 6.3:1 C-85 compression ratio. Once you know that number, you can plug it into the calculation with the longer stroke.

If compression ratio increase runs linear to the displacement increase you could add the displacement percentage increase to the C-85 CR.

If none of this makes any sense, remember I said math isn't my strong suit...

Kevin