Here at Ghetto Shed (with the head on the motor) we'd clean that area REAL, REAL well, with a final cleaning with lacquer thinner, lightly "chase" the threads in the head, install a new (NOT old, EVEN IF UNDAMAGED) stud, with an industrial grade epoxy (I have had great success with Simpson's construction epoxy) spread on the threads, JUST LIKE THREAD LOCKER [on the head side]), let that set for 48 hrs, then "butter" over the outside area with the same epoxy until it's built up slightly larger (bulkier) than the stock area (this epoxy can be filed/grinded after hardened, if necessary). Let it cure for at least 48 hrs. (no time for impatience here), and re-install the header pipe and torque to the light side of the torque spec. with cleaned, lightly oiled threads on the header pipe side of the stud only, AND A NEW WASHER, since this would be a ONE CHANCE ONLY attempt.
BEST OF LUCK!
