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Motor mount bolts typically brake because they back out under vibration and the mount vibrates against their side weakening them until they fail. The good thing in this situation is that if you jack up the motor, remove the bracket and clean up everything to get a good look, there may be enough of the bolt to grab onto or as Brett writes remove it with a punch, or even dremel a slot into it for a flatblade screwdriver... the bolt is usually loose and oilsoaked so should be fairly easy to remove since its not torqued.

Inspect the side of the other bolts you remove before reusing them. If there are signs of weakening, replace with new grade 8 hardware.

Good luck, please let us know how you make out.