Hi eyeofthefly,
did you check for continuity of the field winding ?
You should expect a small AC voltage at the input to the diodes.
Sometimes generators will lose the 'residual magnetism' which
they rely upon to begin to operate.
I have one myself that i have to 'help' from time to time.
I have a small bicycle dynamo, and diode that i can couple to
the field. The diode is to prevent back current when the genny
kicks in, as it will then drive the field itself. I just spin the little
dynamo by hand, and that starts the field ok.
If it hasn't been used for a while, it may well need to be
encouraged to operate. Don't just put a battery straight across
the field, find out which side is plus first, and use a diode cos
when it kicks in it may try to shove current back to your battery.
Usually a small dry cell or two will do.
This is not the same thing as 'Flashing' used on car dynamos.
Best of luck with it, John