If and this is a big if, the rest of the threads are fairly undamaged. You can as a bit of a bodge use an old pedal thread to re-cut the thread in the arm. Cut a couple of slots down the length of the old pedal thread first. This gives the metal that is going to be cut from the arm somewhere to go and creates cutting edges. Screw the pedal in a couple of threads and then remove it to clear the swarf. Progressively screw the pedal further in until it bottoms.
May be worth a try.
If and this is a big if, the rest of the threads are fairly undamaged. You can as a bit of a bodge use an old pedal thread to re-cut the thread in the arm. Cut a couple of slots down the length of the old pedal thread first. This gives the metal that is going to be cut from the arm somewhere to go and creates cutting edges. Screw the pedal in a couple of threads and then remove it to clear the swarf. Progressively screw the pedal further in until it bottoms.
May be worth a try.