Dave Krall asked in 200629 MMDigest, "Is there a way to increase the
output voltage without rewinding the coils?"  In the simple constant-
speed shunt-connected generator, the stator (field) flux density is
already close to saturation so increasing the field current won't
increase the voltage very much.

But you could consider a small DC-to-DC converter like these to boost
the voltage by 2 or 3 volts:

https://www.ebay.com/itm/233523867447 
https://www.ebay.com/itm/402300098727 

Robbie Rhodes
Etiwanda, California