I agree with everyone saying this probably isn't economical to repair by the book but you might get away with a bodge for a bit.
Speculating about the cause of the failure what's that lumpy bit on the front of the broken-off piece? It almost looks like it might be a sanded-down weld as if this was already repaired from some mishap in the factory?
Another possible cause of this is lack of deburring around the hole at the bottom of the slot causing stress to be concentrated and a crack to start.