Measure the diameter of the nut, head down to the hardware store and grab a hole saw bit the same size. I ground down a 3.5" bit and notched it out to match the nut on my 67 c50 15klb 2 speed rear. Worked like a charm.
Hold the bit down flat against a 4.5" angle grinder disc to get rid of the teeth and then notch from there.
Edit: Be safe and use a buddy to hold the grinder while you hold the bit! Safety glasses gloves etc
