All common issues with regard to the broken timing chain guides getting into the oilpan and then getting taken into the oil pump screen, restricting oil flow. Too bad, but fixable. Sounds like your mechanics have all of this under control and are doing the right things. It is refreshing to see competent mechanics working on these cars! I wish we had more of them over here in the US.
The piston rings should be find-able as either NOS parts or new manufactured. It's likely that someone is still making rings of good quality in the proper size. As the
@gsxr suggested, I'd check with Mahle in Stuttgart directly to see if they are selling the appropriately sized rings. There are also NOS parts dealers around Germany, and it's highly likely that an original NOS set can be sourced with some searching.
I would also check with Bernhard in Germany, who is an expert in these cars. He may either have a set of rings, or know immediately where to get the proper ones. Folks here on the forum can provide contact information for Bernhard. I believe he is in the Baden Baden area?