Can I delete my fork before the pull request gets accepted?

Yes, it can be done. From Gitub’s page on checking pull requests locally:

If a fork was deleted, the pull request can still be merged.

Yes, all pull requests are available in the destination repository, even if the fork is deleted before it's merged. The Git tree can be found at the refspec pull/###/head, where ### is the pull request number. The original project's owner can merge using the GitHub web interface or on the command line by downloading the branch:

git fetch origin pull/###/head:pullrequest
git merge pullrequest

I just deleted a fork, and both the open issue and the relative pull request are still there, so I think it can be done.