If it were me, I would trailer the tractor to a machine shop, fabrication shop, or other expert welder. They can probably fix it it, even if they need to add some bracing, gussets, or otherwise.
I am a mediocre welder, but this would be out of my (and most people's) league. Cast is a different animal -- and that's exactly the reason it broke.
Cast steel or cast iron is sort of like a brick. It's strong in some ways, but weak in others.