You could try the non standard socket screw company in Birmingham for new bolts, or otherwise email me with what you need (
neale.shepherd@airliquide.com) and I'll see if I've got some used ones. If the threads are duff what are the nuts in the shell (sills and chassis legs) or the threads in the alloy towers like, if you have to rethread the captive bits you might as well go up a bolt size and change to a standard ISO thread, and bore out the alloy retention cones. A courser thread makes more sense in components exposed to the vagaries of weather and salt anyway.
Best wishes, Neale