If you do the 'drill out' option, bolt a 1/2" plate down with a 2 MM or so hole centered on the bad bolt. This will keep the drill bit from walking. Next a 3 MM, next a 4 MM, etc, Hand file if it's off center a little. Last drill is the bit for a 6 MM tap. Then tap it out. These bolts don't have much loading -hopefully.