Howdy Gil, and WELCOME to the Forum!
First off, do you know of a source for cast iron piston rings in the correct size of approx. 20 x 1 mm?
I suppose that they might be okay, but I'd worry about the cylinder walls getting worn as I'm not too sure what grade of metal they may actually be themselves. I would suppose bronze piston rings could be a compromise choice, but I've no idea where those may be sourced either.
For now I'm content to keep going with the O-rings as provided. I've already gotten a goodly number of spares for very little money, and the originals have an impressive number of hours on them so far. They are readily changed out by just pulling six screws, so it is no big deal.
All that said, if you can source cast iron or other metallic rings for these little engines, I'm sure a number of folks here would be very interested in hearing about it, knowing the source, and getting feedback on how they work out!
Best of luck and please do keep us in the loop!!!