If you can’t get a welder, I wonder if JB-welding something to the inside of the bolts and waiting for the JB to harden for a day might work in a pinch. Also, there exist special Allen bolt extractor tools that “screw into” the inside of the bolts with a reverse thread so they tighten when you turn the bolt counterclockwise.

But they should be used with care since they’re hardened steel, and breaking them will complicate bolt removal by other means. Ask me how I know.