If it's 'cast steel', I'd weld it. If it's 'cast iron', it's a lot more tricky. Might not meet the standard for going to the moon, but a good weld would probably outlive me.
Before having one cast if you can't find a replacement from Fredricks or Hoye then I suggest finding a good weld shop. That arm can be repaired by brazing for sure and maybe by welding. Properly brazed the part will be most likely as strong or stronger than original. I know that if it was my tractor I would braze it myself because I am good at brazing and am confident that it can be brazed as I have successfully brazed highly stressed cast iron parts in the past.
Eric