I recently fixed this stud (S13), here's how I did it.
My stud was intact, but the threads had been damaged. Cut the threads off and turned down the taper until I could drill a big enough bore.
Drilled the entire length out, Bought a 100mm 12x1.25 bolt(this is what the threads are originally too).
Using a counterbore in the origional stud, I used a 0.0005" press on the head of the bolt after rounding off the hex(last picture).
Next step is to put a bead of weld around the head and the start of the threads, then pound that bitch back in my car!
Note: I tried to tap a blind hole in the end at first but the steel (case hardened?) broke the tap!