[NICE 69 S]

I believe there are two different piston crown configurations for 356/912 pistons. Visually inspect the top of the pistons and confirm that they are both the same from side to side. I would also measure the volume of the combustion chambers to see if there is any difference from side to side. Also take the oil screen cover off the bottom of the engine and visually check the camshaft for any lobes that might have excessive wear. The main issue here is the low compression. Even with low compression, an engine will still run without a miss, but will be way down on power. Find out why there is low compression, then deal with the miss.
The gap on the oil ring should have no impact on compression ratio.
As for the miss, if it is at idle, make sure the carbs are balanced side to side and cylinder to cylinder on each side. Then check the idle jets for clogging and needle valve settings. If the miss is under acceleration, check the ignition system and plug wires.
Please note that the copper gasket included in a head set is for cylinder to case sealing, and is NOT a head gasket.

Bob B