I agree with threading it the next smaller size. Iirc, the original threads are 1/2 20 so technically the next smaller would be 1/2 13 due to the smaller root diameter. But, probably sketchy as far as achieving the full torque without stripping again.

Id look at 12mm x1.25 as the best option. You should be able to find a nut with a 19mm hex that would be close enough to the original 3/4 hex as to end up an invisible repair.