Since its just cracked you might want to simply drill all the way through the bell housing and use a longer bolt/nut to keep it in place. Use a copper washer on under the nut and snug/from it to the top of the bell housing. Just make sure the clutch housing is not rubbing it.
If its broken off then I would epoxy it. Cast iron really likes epoxy.. clean the mating surfaces with acetone, let dry, epoxy.
