No you are not going to get a good seal on only 1 bolt for the exhaust pipe that has a hogged out stud.
Check the head carefully. The stud was probably previously drilled for a Timesert, Case-saver, or Helicoil.
Since you reference a M12 size hole (almost 1/2") that seems like it is far more likely to be some sort of Timesert or Casesaver. Are you sure it is even tapped metric? DAPO may have tapped SAE. Just sayin'
I'd be surprised if you can get a M12 bolt though the exhaust pipe flanges or if the head has enough meat in the stud area for a M12.
Proper fix is to remove the head, weld up the hole and re-tap.
Alternately, if you can figure out what the head actually is tapped for, you can have a machine shop turn up a special stud. Say M12 on the threaded end into the head, and then M8 on the end that will go though the flange.
I'd guess the right shop machine shop might do it for about $80 for a custom stud. Or find a friend with a lathe and provide the beer! No more than about 1/2 hr of work. Likewise, a machist could do a SAE on the head side (if . . .that is what it is) and M8 on the flange side so you don't have to mix and match nuts.
Would be cheaper than head removal and welding and would be a reliable fix.
If you can't afford either of those fixes you definately shouldn't contemplate a six.
Can't help you on the meaning of life - I've too have been stuggling with that one lately, given how crazy the world has gone.