They are held in with kind of a sheer nut thing, you have to drill them out, then find some way of fitting the new one. I had to use rivets on a replacement one I did a while back, it seems to have held up well. You could use rivnuts but you have to drill bigger holes in the bulkhead.
Drill where the holes are just about visible in the bracket, it wasn't big, but very hard steel, I started at 2mm and worked my way up to around 5mm drill bits. When you get through the head of the bolt/sheer bolt thing and the old bracket comes off you are left with stubs, bang them into the bulkhead with an hammer.
Or , just get the broken bit welded back on.
