You can’t restore a user once they are deleted. There’s no avatar for those posts now because there’s no user account.
There are a couple possible reconstruction solutions to restoring a deleted user, but they involve installing additional software tools (which I might not be able to do on our server) or running some sketchy SQL database commands that I found, but those are dated 2009 and are probably out of date. So they might screw up the whole database. It would take me a long time to figure out and test all that.
Bottom line, not really worth fixing for the vanity avatar 🙂