Mr. Krall,  Although I know very little about Tel-Electric pianos
I just thought about the use of a Variac adjustable transformer as an
answer to your problem.

Depending on the supply frequency in your area, and if the piano is not
equipped with a synchronous electric motor for the roll transport, try
a transformer-rectifier.  Put a large Variac variable transformer at
the power line followed by a suitable industrial-use rectifier like the
ones used in arc-welding machines.

That way you would be able to reach the desired DC voltage.  This
should remove the need for rewinding the generator since you won't use
a motor-generator apparatus anymore.

I used some large Variacs that way in some pipe organs requiring odd
DC voltages at high amperage and it worked very well.  I would send a
picture of one of them but I think photos are not accepted by the MMD
e-mail system.

Gilles Chouinard, organbuilder
Laval, Quebec, Canada