You can arc weld it with nickel rod but you must v groove the crack and I'd recommend drilling an 1/8" hole at the end of the crack. Must be super clean then preheat with a torch to at least 500 degrees weld it from the end of the crack to the 1/8" hole. If the crack goes all the way through the case you'll have to disassemble everything because the heat will draw the lube out through the crack and ruin the weld. After welding wrap the case with fiberglass insulation to trap in the heat and allow it to cool very slowly. Done this many times engine blocks also the same way.
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