Your best bet is to try and find a way to knock the bolt heads off, remove the shrouds, and then attack the problem of the spinning insert.
I had this exact problem on my KX some years back. I labored for HOURS until finally I just cut my rad shroud off and could finally fix the insert. I ended up drilling out the threads of the brass insert, and pop-riveting a small flat rectangular piece of steel to the blown out insert. To that piece of steel, i tapped some threads to hold my radiator shroud back on. I had to drill a new mounting hole to my rad shrouds because the threaded hole I made was slightly offset from the original mount. Once finished, the fix worked wonderfully, and could hardly be noticed.