Here is a link where a guy is having similar issues with the same model gen.
It mentions what the resistance of the rotor should be, slip ring to slip ring as 55 ohms.
AND other tips.
I searched for a SERVICE manual online and came up short.
I thought maybe Clymer might have such but came up with a dry well.
You mention 500 ohms as the rotor resistance. (make sure you are actually reading your ohmmeter correctly) Should be 45-55 ohms maybe. If it is 500 ohms might be time to throw in the towel.
If you are seeing 500 look for a bad connection maybe at the slip ring. (this will be the only place you might be able to see each end of the rotor winding connection)
I would suspect you have a bad stator or rotor with a low resistance instead of high.
He also indicates what the voltage should be when 12DC is applied to the brushes and the gen normally has 115v DC to the brushes when normal for 120vAC at the receptacle.
https://www.askmehelpdesk.com/electrica ... 38698.html