I am sure Remington has a distributor in Australia. Can you send the gun to him either for repair there or return to Remington? It might take time, but the job would be done right. Because of headspacing concerns, they won't take just the bolt - they would need the rifle.
Most of the weld-on/screw on bolt handles depend on the stub being present. If the handle was broken off at the brazed joint there is nothing to drill and tap into and welding might not be a good idea because of the possibility of ruining the heat treatment of the cocking cam.