I have used J-B Weld, sold at auto supply stores, on some broken
castings with success.  Welding the pot metal, that old metal parts
in a player piano are usually made with, takes a _really_ expert
welder and you need a spare part because it will probably be ruined
with a "I did the best I could" explanation.  On cast iron pieces
we have a welder that uses a "spray welding" process.

Don Teach