I like the idea of using a V-twin compressor for the foundation of a stirling. B-)
If the rings have a lot of friction perhaps they aren't properly lubricated after the compressor sat around a while. There should be little friction there or the compressor would have burnt out, much like an engine with a bum oil pump or low oil.
How hot will you be running it at the piston level? Can you get away with replacing the rings with something like delrin?
Before removing the heads and removing the crank shaft I noticed a lot of air entering and existing the crank case when I was spinning it by hand. There is a lot of back pressure caused by the pistons.
I think that, when it's running as a compressor, that air acts as a "spring" to store power in the down part of the cycle to help on the up part. This evens out the load on the motor. Your stirling will probably not be self-starting if the crankcase remains sealed. If you vent it, be sure to minimize the airflow restriction, which would be a power loss. Beware of evaporating out any lubrication with that air.
I am going to try to replace all the bearings with sealed bearings and leave the crank case open. This should remove some of drag and maybe the it will run using dry lubricate and the original Piston rings.
The cylinder heads have two ports. Both can be used to pass the gas between the cylinders. Using a dual regenerator configuration should reduce drag and may help the Stirling cycle.
Or just hook 'em to a common regenerator.
You could also put in an extension on the cylinder with a big port. (You'll probably have to put an extension on the displacer cylinder anyhow.)