Replace vent cover or add mesh screening

Recently we’ve had some mouse activity so we’re on a mission to find and seal up any possible entrances. Looking at this kitchen vent which is suspiciously located near where the activity is happening inside, we found that it does not have a screen on it, just a simple flap which does not appear to be doing a very good job.

I purchased both a replacement cover and a some metal mesh, but now I’ve run into the problem of how to actually install one or the other:

First: The two screws on this existing cover appear are screwed into the mortar in the foundation, and will not budge even a little. Without removing them, I can’t possibly install the new vent cover.

Second: Failing that, I have my backup wire mesh, which I can cut to the dimensions of the existing rectangular opening, or possibly jam up under the existing cover, but how I would attach that to the current fixture?

Vent

Inside of vent, showing the futility of the flap