I feel like given that you opened an issue and there's been no response--and presumably, that issue is visible and thus on record, as it were--that it'd be fine to go ahead. I'd even consider following up on the issue and saying that you've gone ahead since you didn't hear back and the repo was accessible, etc etc, here's the link, and certainly mention the original repo in yours and all of that, but that's it's basically fair game at that point.
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I feel like given that you opened an issue and there's been no response--and presumably, that issue is visible and thus on record, as it were--that it'd be fine to go ahead. I'd even consider following up on the issue and saying that you've gone ahead since you didn't hear back and the repo was accessible, etc etc, here's the link, and certainly mention the original repo in yours and all of that, but that's it's basically fair game at that point.