The shape and weight of the "toilet bowl plunger" at the end probably is the most important thing to be preserved to keep the original sound. The rod it hangs on was probably cast and then hammered (wrought) to make it strong enough to hold the knob.

A re-casting of the arm will get you plain old cast iron. That should be okay for the knob on the end, but the rod needs to be "wrought" by heating and hammering to break up the grains of the metal. When iron cools in a mold, the part that gets hard first has a different composition than the part that solidifies last. It has to be "wrought" by hammering to redistribute the components of the mix, sort of like working a bunch of raisins into your cookie dough to get them uniformly distributed. Even if you end up with the same shape you started with, more or less, the hammering is important to getting good strength and toughness in the final part. Almost any experienced blacksmith should be able to advise, and to do the job, although few are likely to have the facilities to do the recasting.

Welding wrought iron is a little tricky, since the weld will be "new cast" material and you can't really work it much. If a welded fix is chosen, it would likely be desirable to "add on" a couple of sidebar pieces to span a few inches past the edges of the break. Of course this will change the appearance of the rod, but it would take a fairly large amount of added material to affect how it works.

The suggestion to contact the bell foundry for advice is a good one.

John