Rob
There is a special low amp burning stainless rod and low amperage welder that you stich weld with
do it in 1/4 to 3/8 in. welds and peen while warm let cool before the next stich
all preheated to about 200 F
best bet is to use a heat gun to control the temperature
a long slow process but it works
the whole trick to welding cast iron is don"t rush
you are basically peenin the rod into the crack while the cast iron is warm and the crack is opened up
Stew