Hi,
I know the thread is kinda old but it might help what I have on this subject.
I asked a supplier in Europe and the part has been discontinued so this route is a dead end.
I know these models use the same part as your MD-SR50
-MD-MT50, MD-SR60, MD-SR70, MD-SR75, MD-MS721H and MD-MS722H.
I'm in a similar boat because my SR70 has the gear with bent teeth and I'm looking for a replacement.
I hope that helps in any way.
-KorgerXP