There's one I know of that's decent. Before I bought my own welder, I had them do a few things and I was happy. I have never asked them if they weld cast iron. I understand it is difficult to weld since cast iron is porous and lower quality metal. I'm thinking it should be easy for an experienced professional.
I kinda liked Jimynick's idea of heating with a torch and then using my mig on it. Problem is I don't have a torch.
I guess I could take the cap into that shop and ask what they think. If they were confident of an easy fix with no warp issues, then great. The more I think on this, the more I'm convinced it could probably be run as is without any issues. If the weld shop doesn't give me good odds, I might just do that.
Good thing is the guy didn't want to buy it with this crack and since I don't need this engine for a project anytime soon, time is on my side.