I've used Viton in a number of CI cylinders with no problem, just give the cylinder a Home with a cheap brake cylinder hone to remove any turning ridges.
Don't use the makers figures for ring compression as they will create too much friction.
If it is just a 10V or 10H then you don't really need rings, just add the oil grooves as shown on the drawings, 1000s have been built that way.
Edited By JasonB on 01/02/2021 16:21:08