Options to repair rust in Duster fresh air vent

I added two pictures in case you are interested to see what the Duster air vent system looks like with the upper firewall removed. I'll deal with replacing the firewall later - right now I want deal with the source of the water leaks.

The rough looking material that you see around the vent is apparantly some kind of mastic material that was installed at the factory. It is actually in better condition (less rust) than I expected it to be.