I feel your pain, I broke a fin on mine when cleaning it, I have trouble with my thumbs and can loose grip suddenly... this was one of those times. I drilled and tapped the fin below it and bolted it to that one, packed out with washers. With it all painted and fitted you’d have to look for it to know.
To add, when I worked in garages, the only cast iron we welded was exhaust manifolds on Peugeot 206’s. We used stainless steel for welding with. Would never do it with standard steel MIG weld wire. ideally you would heat the whole item first (Then post weld also I think) and then allow to cool naturally to normalise the weld. We didn’t do the pre heat process with the manifolds and they never blew again afterwards. But MIG welding stainless uses much more amps (heat) than standard steel mig wire