Dave, why do you want to use an incorrect motor generator?  Is it
to look original?  To increase the output voltage you will need to
either increase the speed of the generator or rewire it.

If the motor is an AC motor you can change the speed by using a
variable frequency drive.  These are readily available now and are
not expensive.  You can hide the drive where it will not be seen as
an un-original looking device.

To increase the output voltage of the generator 20% will require
a similar increase in frequency.  Most AC motors should be happy with
this small an increase.  Increasing the frequency a lot can be asking
for trouble.

Spencer Chase