According the the manual that I once had, all you had to do to
re-magnetize the fields was to momentarily hook a 6 or 12 volt battery
across the output leads (120/240) for a second or three.
This is not correct.
To flash the field, a 12v battery must be connected between terminals F1(+) and F2(-), not the output terminals U1 and U2. The battery must be connected briefly while the generator is already rotating at rated speed. You should see the output come up to rated voltage almost immediately as the battery voltage is applied at the field terminals.
Again, if the generator was working when last stopped, I don't think this is a residual magnetism problem, especially since the OP already stated he tried flashing the field. But it could be, and he might have done something incorrectly, such as reversed the battery polarity.