I wonder if an Allen headed bolt would even work for your application because the diameter of the head of an allen bolt is 3/4 inch. The original bolt hex head is 5/8 of an inch across the flats. I would think you would have to grind the side of an allen bolt head to even get past the engine case. Not only that the height of an allen bolt head is 1/2 inches. So once the allen bolt is set you would have to make sure that it wouldn't turn and interfere with the engine case. The original hex headed bolt is somewhat special because it has a reduced head size that would use a 5/8 inch wrench instead of the standard 3/4 inch hex. I would be inclined to go with the original Andover-Norton engine mount bolt.
What I found that works very well to hold the original bolt head from turning is to use a modified deep offset 5/8 inch box end wrench. The modified wrench has a ground taper on the top part of the wrench so that the narrowest end is about 1/4 inch high. Here is a side view of the finished wrench:
Engine Bolt Wrench Side View
Here is a picture of the wrench
in action:
Engine Bolt Wrench