Hi, Dave,  the first thing I'd do is try it on 12 volts.  The
label may say 14 to 15 volts but there might be enough of a
'safety factor' to have it work on 12 volts.  The same with an
original motor-generator.

If that doesn't work, I'd try the opposite approach.  Run the
motor off a Variac and set the voltage a few volts above 115 VAC.
That should speed up the motor enough to boost the output DC a volt
or two.  That should be fine as long as you don't overwork it.  The
motor designers build in a safely factor, too.

Another approach would be to build or buy a 14- to 15-volt power
supply and hook it up in parallel with the motor generator (with the
M-G wiring disconnected).  Of course, you wouldn't get to hear the
whir of the motor mixed in with the piano music any more.  In fact,
if you can find an appropriately shaped motor, you could even build
the power supply _inside_ the motor so it would look original.

Regards,
Craig Smith
Upstate New York