Magnesium can be TIG welded. I had exactly the same break on my 83 XR5 stator cover and managed to weld the broken piece back on with a 'poor man's TIG' setup. That's a arc welder, TIG torch/Argon plumbing, and a high frequency adapter. I cleaned the heck out of the pieces and used a bolt to clamp the broken piece for welding. I used a pulsing technique by turning on/off the HF adapter and left the gas flowing, no filler rod. I did this repair on the bike after removing the gas tank and draining the carbs. Covered the exposed portions with a fire resistant welders blanket. Not exactly easy, and my repair isn't the best looking on on the planet, but it can be done. Alternative is to make a bracket (some folks have even used a washer of the right size and screwed it onto the remaining mount. Or you can buy a bracket from XRsOnly.