I have done this repair several times now on 123s. An earlier post mentioned that this should be structural because the seat belt rides on the seat. That is true, but in my experience with rust in that location, the seat belt where it attaches to the inner rocker is also compromised. If its rusted out below where the belt attaches, it will need to be welded to be safe.
If the metal around that seat belt attachment point is good, I would say you can get away with por 15, sheet metal, self tapping screws, and some kitty hair fiberglass. Clean it up and find the maximum extent of the rust, put several coats of por 15 on it. It looks like the seat mounts are completely loose, if not tear them out, clean and por 15 if they are solid enough to reuse.
In order to be strong enough to hold the seat, you will need to bend the sheetmetal and screw it to the inner fenderwell (in front of the seat belt bolt hole). Then bend it into place and screw it down as best you can. You will need to screw the seat mounts to this new metal so you have a place for the seat to mount.
Once you have it as best you can, use the kitty hair fiberglass to waterproof and seal it from above and below.
Bolt your seat in and go, or cover the whole repair with undercoating for the "professional look".
The above is redneck body repair 101. For a more authentic redneck look, substitute sheet metal with license plates, and the kitty hair fiberglass with caulk. I would caution against that, though as license plates don't last long as structural components.
