A friend and I fixed a couple of teeth on his Logan bull gear a few years ago. From somewhere or other he got another broken bull gear and sawed out a block containing two or three (whatever it was) gear teeth. Then he made a notch in the gear he was trying to fix, and we brazed in the block. Years later, it's still working fine.
Having done that...I think if you can build up the broken teeth by brazing and then machine/file new teeth, you'll do just as well if not better. Just take your time and try for fit, meshing the gears by hand, until they run smoothly.
Another approach would be to cut slots, braze in slivers of cast iron, and machine/file those to shape. I recently fixed a gear on a tower clock that way. (It was brass, but same technique.)
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