Get a very good standard blade screwdriver bit, use one of those ones you can buy for a buck or so at the hardware store. It should be just about as wide as the head of your allen screw, but not quite. Or, just a little bigger than the gash you left of the old hole. Working carefully, sharpen it down to nearly a knife edge, and then v-in the corners just a hair or two.
Using pads to prevent marring, chuck the scope into a vise, and after mounting the bit in a solid handle, place the bit across the screw at a place where some of the old hole edges remain, if any. Using a brass, or other solid hammer, tap the bit down into the screw head, basically forming a new channel for the blade. You need to give it two or three good blows, making sure to line up the same each time. Now you have a slot, use the bit as a screwdriver again, put lots of downward pressure on, and use a visegrip on the shaft of the screwdriver handle to apply turn.
Then, next time, take it to a gunsmith to mount the scope, hahaha!!