Hurst V-gate repair advice needed

It broke at the thread. It will probably do it again. You can get it in a 4 jaw Chuck, indicate it in and make it round, but that doesn’t solve the real issue.

And that is you have way too much leverage with that long grip. You have to reduce that leverage or make the threads probably double the diameter. Having the grip that far above the threads makes that last thread (the last thread below the lock nut) a pivot point and it will break again unless you address that issue.