if any of them still retain the head and aren't stripped, you could try heating the head, wait to cool, then try it again. for the ones without a head, i would drill all the way through the bolt, starting with a small bit and increasing in size each time you drill through it. repeat this process until you have ONE size larger then the original bolt. you will have to thread the inside again, but that's not hard, just go out and buy the same size tap as the final drill bit you used. you'd obviously have to buy new bolts as well. this method is pretty time consuming but it's what i would do.