Captured metal plate that is threaded, not sure on the proper name but I know the bit you're talking about. If all else fails, you could cut the side of the "frame" section to make a piece that hinges up, and fish in the resulting hole to pull out the captured piece. Soak it at least overnight in a small tub of PB Blaster or another proper penetrant, then do as you sig suggests, and get the remaining bolt stub out or take it to a metalworking shop and have them copy it for you. That would probably only cost $25-$30 at a more expensive shop.
Once you get it fixed/re-created, slip the plate in and put the bolts in to ensure correct positioning, then bend the little flap back down and weld securely. I'd also weld a patch plate over the whole repair area, but that's at your discretion. Make sure you don't light anything on fire above the floor, or yourself laying underneath it.