The Sega Genesis game Road Rash only has a single moderator, and has not signed in to the site for the past 2 months. I have no idea how many runs are pending at this point (realistically not very many for this game), but I have had a run pending for a few weeks now.

Is there a process for replacing a game mod if they vanish?

I would volunteer to mod this game, but I haven't been able to find a location where it seems appropriate to volunteer.

https://www.speedrun.com/road_rash

First, comment on one of their YouTube videos and message them on discord asking them to review your run. If that doesn’t work, and you’ve had a run pending for three weeks, you can request to become mod through the support hub.