Your bell is probably cast steel and not cast iron. Cast Iron is crystaline and very brittle and has a clunky tone even without a crack. Cast steel is less brittle and has a sharp piercing tone. If it is iron, there is little that you can do to salvage the bell as welding will not improve the bell or its sound, and may result in its shattering into many pieces. If it is cast steel, you can try welding the crack or braising it with bronze braising rod, but it is unlikely that the weld will hold up and the tone will remain poor. My best advice is to keep it for ornamental purposes only. If you need a functional bell, you should look for a replacement.
Harry Long, MD