There have been a few enquiries recently for rather specialised internal parts for engines - piston rings, rockers etc. First, I don't want in any way to undermine the Club Spares Scheme which is always likely to be your best first port of call, but that broadly supplies Reliant parts, which as we all know were usually supplied as modules, or assemblies. I don't think you'll find any R-Part number for an individual piston ring!
I am not endorsing them at all, as I have never used them, but they seem to cover a pretty broad range of the sort of smaller parts that the R-Parts catalogue overlooks. In particular, have a look at the Part Numbering and Interchange table they illustrate on the link I've given - it expands if you click on it.