You take a grinder and chisel and knock out mortar in a step type pattern and then put copper (or crap) into it. I grind my verticals too, no caulking needed. It is not something you will do yourself and make it nice on the first shot; hire a pro…just not the one that did it the first time, heh.
NOW…the real bitch is I bet the existing aluminum is a through wall flashing. Meaning it goes all the way through the brick and was laid in as they did the wall. Problem now is that you won’t be able to get all of it out and technically copper is not supposed to touch aluminum because it causes a galvanic reaction. So you may want to opt for lead if you can get it. I could do that wall in copper but Id charge you triple for the time it would take to work around the original reglets.