Is This Exhaust Manifold Repairable?

I'd give it a shot- as said, clean out all the old braze repair, preheat and prolong the cooldown. Nickel rod, or Muggyweld makes specialty rods just for this particular type of repair. Or leave it to a professional.
I think the real problem is going to be finding a machine shop willing to do a one-off setup and mill/blanchard grind the mating surfaces flat again, it's getting hard to find someone to do piecework anymore.