Can you add to a Github pull request after the source repo is deleted?

I’ve opened a pull request (PR) on a repo, but then managed to accidentally delete the branch I initiated the request from. The maintainer of the project has requested I make a couple changes.. now I’m stuck.

Github still has the PR, but now shows the source of the branch as “unknown repository”.

I was able to recover the exact commit and re-push that (it has the same commit hash as the commit in the PR), but it still shows as “unknown repository”.

Is there a way to push directly into the PR itself?