I'm assuming the heads broke off and that the remaining bolt is roughly flush with the frame and that the cause of this was corrosion. You're probably going to have to drill the bolts out. Start with a small size bit and work your way up. I'd use cobalt bits as they'll make this easier, and make sure to keep the bit lubricated with oil as you're doing it. If you see smoke while drilling, your rpm's are too high. Keep the bit rotating on the slow side.
Get replacement OEM bolts. Corrosion means that even after drilling, the bolt might not come apart on its own - you may have to drill the full bore and re-tap the hole. Find out what size bolt it is and buy a tap kit for that size, it shouldn't be all that expensive. The kit will include the proper size drill bit and the tap to thread the hole.
I had to go through all this when I removed my exhaust flange bolts. Two broke and nothing would get them out. Eventually drilled, re-tapped, and put in new bolts with lots of anti-seize.
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