Usually for other repo, fork it and use that forked version with the necessary updates.
But BTCPayserver plugins seem to be packaged by their own plugin builder:
docs.btcpayserver.org/Development/Plugins/#publishing-the-plugin to be published in their plugin list.
If they allow forks (
DYOR), you may try to release your updated version if the author is really inactive.
The thing is, I'm kind of a repo purist. I don't like forking software unless absolutely necessary, like if the software is woefully out of date and there's like 20+ changes that need to be made. Lots of repos with 1 contributor lying around I feel pollute the ecosystem - I myself have a bad habit of not maintaining many repos I end up forking.
Gregory's repo is already on BTCPay plugin directory, so I'm going to try all my options to get that updated somehow first.