I've done quite a few of them. I use a tig welder and flat welding ceramic plates. You slip the ceramic in place under the broken fin and run weld beads letting them rest against the ceramic. Just build it up a little proud and after it cools you remove the ceramic and grind to shape. You can also sand blast the repair area to blend it back in with the cast surroundings.

This will also work with mig but you'll have to get pure argon gas and not the standard 75%-25% Ar-CO2.