How do I replace a bricked in mailbox?
Is it rusted enough that you can pull the old one out?  If so, you might be able to find a similar model that could slot right back in the hole, possibly with a little modification.  A plastic one might be a good candidate, both for ease of modification and for weather resistance.

If you do manage to do this, I wouldn't try to mortar the new one in, but instead hold it in with Tap-Cons or similar self-tapping concrete screws.  A few of these could be driven into the inside brick surface through the mailbox shell, just to keep things from rattling around too much when you open the door.