Unfortunately, there are two features of the old piano motors that
are not likely to be found in new motors.  (1) The old motors were not
as efficient as new ones but this allowed them to hum quite a bit less.
New more efficient motors will be found to hum quite loudly compared to
the old ones.  (2) The old motors also had sleeve bearings whereas all
but the cheapest low quality modern motors have ball bearings which
produce a rumbling sound.  This is not true of the highest quality
bearings so it would be possible to replace the bearings and eliminate
this problem.  The hum would still be there and the fact that the
motors are different shapes and mount differently means that
modifications will be necessary.

It would probably be possible to use a high quality brushless servo
motor and driver.  Since the motor would be much smaller than the
original, it might be possible to provide a well-designed resilient
mount that prevented transfer of motor noise.  Not counting the time
build such a contraption, I would guess that the components would add
up to about $600 or so.  A brush type motor would also work very well
and cost considerably less but brush noise would have to be muffled
somehow.  If anyone wants to try this, I highly recommend motor drives
by Gecko Drives.

I also found a very good and inexpensive encoder and some reasonably
priced servomotors and transformers for power supplies.  In addition
you would need something to give pulses to the driver.  You can also
buy inexpensive DC motor controllers from Amazon and not bother with
the servo driving stuff.  Just be warned that any modern motor is going
to be noisier than the old ones, unless it is very expensive, so you
have to provide additional noise isolation.

Installing a remote pump and modern motor seems like a much better
alternative to me and is what I have done with all of my pianos.

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