It doesn't hurt to publish, and others will benefit. Make the fork public (and be sure to document the breaking changes), then reach out to the upstream owner t

It doesn't hurt to publish, and others will benefit. Make the fork public (and be sure to document the breaking changes), then reach out to the upstream owner that you'd be happy to advise on your changes if they want to consume the effort to merge back and maintain their current compatibility. But as you said, the project is unmaintained and you likely won't get a response. If you don't get a response, then the world can benefit from your fixes.