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I have built up teeth on old machine tools, but these were irreplaceable soft gears and not subject to more than a few hp. I do not think this is a good repair choice for a couple of reasons. First off, you really need to pin the missing teeth with steel pins driven into holes so the braze has some reinforcement. If you don't pin them, they will probably pop right off the base metal. I like to install pins that are the same diameter as the flat on top of the tooth to get max support. The pins need to be straight in a line and EXACTLY in the center of the tooth, as well as perfectly square for this to work. This will be difficult to do accurately with a hand drill. Second, that gear is probably forged, as mentioned, and is also hardened. Putting a torch on it is going to ruin the hardening and may release internal stresses, causing it to distort. Add to this the fact that you will spend hours on end trying to file the new brass teeth to shape. If you have never filed brass you are in for a real treat. Brass will just laugh at a file that is even slightly dull, the file will just slide across the top. You'll need brand new flat and knife files to go at this. You will have to blue the existing mating teeth and file until the desired pattern (a nearly full covering oval in the center of the tooth flank) is achieved. You blue the teeth, file, wipe off the blue, blue again, file, etc... Now, if you can get the gear out, you can put it in a dividing head and use a horizontal mill or shaper to cut properly shaped teeth. That would be a best way to do this... BUT. As you can see, the amount of chips and crud generated by the drilling, brazing and filing that will take place would be far better done with the gear out of the tractor... which was the whole point to begin with. By now, you could have just pulled the tranny and installed a new gear. You'll never have to worry about it breaking again and it'll fit right out of the box.
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