I guess the answer would lie in knowing how it is put in there . is the stud in a nut that is welded to the inner metal plate/inner frame? or is it a stud that is welded to the plate itself?- so how to put a new stud in??

and is theis a OEM bumper top?

Liiks to be OEM.
Epoxy and new stud after you drill out a cavity to pot a new stud into

the easiest thing to do would to grind flat, drill a smaller hole and tap and install a smaller bolt with a washer

I've done that twice!

How far do the threads go in? There is a woodworking tool called a plug cutter that acts like a core drill. If the threads go long enough, maybe a threaded sleeve could be used to join a new stud to the old one.

great idea but its too short.

I’ve done it before drill and trap it with 6/32 trap and install 6/32 machine
Screw works great ! Be careful with the trap it’s small and will break easily
Go back and forth with it !

Use a same sized bolt not stud, I did this years back to a top and the fix has never failed. A little black plastic disc at the spent stud will pop out. There's a write up using the search.

The problem with the smaller bolt is that the reason the old one is missing is rust. Safe bet much of the substrate is rusted also. Ypu need more surface area to retain the stud, thsts why I use the potting method.
We also aell the rubber washers if you need them.

i have the runner washers from you Mark, i will get this done in a few weeks, not the big priority, the bumper it gets put on ison the way to the chrome shop now, other priorities ate the ss gas lines, thanks all for the suggestions, i dont see any rust inside this bumper top is almost like new, it was just that the nut had rusted to the stud and snapped taking it off.

The foam rubber holds moisture. They rust from the inside out, just like the cars. I don't think you want to put too much work into this because it will only get worse over time. Not as much now that it's not a daily driver but you cant reverse the problem.