If you can't find a new plate, I would try to salvage the old cracked
plate.  I have had excellent results from using pure nickel welding
wire or rod to repair cast iron steam radiators, several engine
manifolds, and quite a few other broken castings in general.

Remove all the strings and tension, remove the plate from the piano,
grind a Vee-shaped groove all around the length of the crack, top,
one-third depth.  If it's convenient to do so, I'd preheat the plate
at the crack with a blow torch, or some sort of gas flame (it doesn't
have to be red hot). 

Rotating the plate as needed, weld the crack always in down-hand
position, welding long side, opposite long side, short side, opposite
short side, adding additional weld filling as required until you can
cool the plate for a day, and grind and fill the shape to match the
original casting profile and make the weld sort of disappear.  Fill
any remaining gaps or holes on the profile with epoxy (or Bondo) and
go on to paint finish.

Good luck in finding a plate, but if you can't find one, I'd sure try
welding, and I'll bet it works.  I'd be interested to learn of your
results if you have to weld.

Jack Hardman