The good professor Jim has already posted my long winded Troubleshooting Procedure but basically (If wired right n all else okay) if the genny is okay the problem must be the Volatge Regulators are bad (but I doubt if two in a row are) OR THE VR ISNT WELL GROUNDED run a jumper wire from a good frame ground to the VR n see what happens??????? Make sure the wire from FLD on VR to FLD post on genny is good.
Sometimes a genny shop will have the genny polarized at Neg ground (and you are at Pos ground likely), to be safe Polarize it on your tractor n see what happens!!!!!!!!!!!!
After you Polarize do what my Par 5 says and as posted below and dead ground the gennys FLD post n see if she charges then?????????????? If so but NOT otherwise the VR isnt well grounded or is bad or the wires bad from FLD on VR to FLD on genny
NOTE if an ammeter is bad/stuck but still continuous, even if it dont register charge ifffffffff shes charging at fast RPM the battery (if still good even if somewhat discharged) voltage would rise from 6.3 setting to at least 6.5 and around 7 volts Does battery voltage rise (or lights get brighter) when shes running at fast RPM????? if so shes probably charging