To add to the M6 vote I’ll say something to maybe fix your situation.
You can’t get a bolt back in that spot, but you can cut the stripped bolts off. You’ll have to cut new M6 all thread to the perfect length and use something to temporarily support a nut inside that fork leg while you screw the threaded rod in from the outside. Then proceed to use as normal.
Way more tedious. So long as you aren’t taking that stabilizer-minifender off very often, it shouldn’t be a pain more than once.