Little late to the tread as it seems the Op took the route of buying a new pulley, which is what I would have done. Anyways, I have had pretty good results welding casted pieces using mig and regular wire and gas mix. Preheat and Post heat is key. I use a propane weed burner on large parts and the top of my wood stove on smaller pieces. I peen using my needle scaler. I like the part brushed really clean and line up the pieces on a metal table where I can run the weed burner over it. On something like that pully, I would preheat to around 600*F using a infared thermometer to check temps with. Then I would use my mig to spot weld several areas around the break just to keep it lined up. Each spot weld would be post heated as soon as weld is complete and then peened with the needle scaler before going to the next spot weld. Once everything is spot welded and no crackes. i would start making small welds, no more than about a 1/2in on such a small piece, post heat and peen, continueing around the piece until the entire part has been welded. Even after the weld is complete, I would apply post heat and keep checking the temp making sure the entire part was the same temp and then cover with something to let the piece cool down very slowly. I like doing these type of repairs in the winter time because I can just lay the part on top of the stove and let cool as the stove fire dies out overnite. If you get in a hurry and try to weld everything up in on pass, it most likely is going to crack. Go slow and skip around a bunch and you can get it to work without cracking. Also, I think brazing a pulley like this one would be easier and much faster than trying to weld, and might possibly be even stronger.
