What was the "Decaying Foam" problem?
They were assembled with low-density "foam rubber" (probably synthetic) wrapped around the prism, and it deteriorated over time. The chemical products etched the surface of the prism. The repair involves replacing the prism with a scavenged one (the OM-10 used the same prism, so there were supplies).
Here's a page from a specialist repairer with a photo:
They stopped using the foam some time into production. That's it - no other changes - the foam hadn't even been necessary. My OM-2N never had the problem either. My OM-1 did eat a prism (and its mech jammed later, worn out beyond economic repair).
The later OM-4, OM-3 and OM-2SP were a newer, more longwearing, design.