If the Holtzer Cabot 60 Hz motor is of induction type and can be wired
for 230 volts, have you tried connecting it to 50 Hz mains?  It should
run happily at just under 87% of its rated speed, which may be adequate
to operate the piano.  Otherwise a proportionately larger pulley on the
motor spindle would probably be cheaper than a frequency converter.

I have an American 'Century' 60 Hz motor which was re-plated by the U.K
importers to 40 Hz without other modification;  it drove an organ-blower
for at least 50 years, latterly on 50 Hz.

Barrie Land, UK