wow... rusted and gone ???..... amazing.....
that area should be accessible from inside the car - not ??
I would think the plate (bracket) is glued and riveted. If I had to install a new plate I'd clean the mating surfaces, roughen it with 80 grit and glue the plate in place with some structural adhesive like 3M 8115, 8116 or the 3M adhesive that comes in two tubes (I bought that stuff on Amazon, it's structural adhesive for frames), it's smaller quantity and cheaper than buying a cartridge 8115, think I paid $20 for the glue.
Once the glue is cured I'd use bolts, washers and locknuts instead of rivets - not NCRS correct but that wouldn't be a concern for me