I would apply liquid wrench or pb blaster between the outer ring and inner metal piece. Let it sit a day or two then try side cutters or small vicegrips on what remains of the studs and try to pry out.
It worked on a couple of mine.
I then drilled out the studs and brazed screws in.
It worked on a couple of mine.
I then drilled out the studs and brazed screws in.