Greetings,  I have struggled with the motor replacement problem and
have found no cost effective substitute for the original motors for
several reasons.

The original motors, although less efficient, are quieter than all
but the most expensive specialty motors made today.  It is not just
a matter of balance but also of hum and bearing noise.  It is difficult
enough to make the original motors quiet and next to impossible to make
a modern commercial one even close.

The original motors used sleeve bearings which are quieter than ball
bearings with the exception of very expensive precision ones.  Motors
made with sleeve bearings today are generally the lowest quality and,
although the bearings might be quiet, the rest of the design is not
something you would want in a piano.

Grand pianos require motors with the shaft vertical.  Most motors are
not recommended for this unless they are vertical shaft pump motors.
You can modify a horizontal shaft motor but it will still be noisy.

It is my opinion that only two solutions are reasonable: (1) install
a remote pump and use a modern noisy motor that will run full time and
not cause a fire as they have thermal protection, or (2) rebuild the
original motor with higher insulation class wire and new bearings and
start switch parts if necessary.

It is difficult to imagine that a simple motor like these can not be
fully rebuilt for $500 or so.  I was quoted a price of $200 to rewind
the original Welte turbine motor and that is a much more difficult
motor to rewind.  I have not had it done but should do so before they
change their quote.

Best regards,
Spencer Chase
Garberville, CA
http://www.spencerserolls.com/