You could try the "model T" method of sealing it - start at one end of the crack. Drill a hole and thread it. Insert a plug in the hole (one of those with the square indent made for a 3/8" socket drive extension to fit into or perhaps one with an allen wrench indent - important thing is that the plug doesn't have a head). Move over about 2/3 to 3/4 the width of the plug (away from the indent used by the tool to tighten the plug) and drill the next hole, drilling a portion of the existing plug out. Thread and insert another plug. Repeat until you've covered the entire crack with plugs.
Of course, your welding ability on cast may be such that you would weld instead of trying this. My cast welding ability would make me either try this or take it to my local machine shop ...!