Do you have the Neihoff manual? Goto the neihoff website here to download it. There are troubleshooting procedures in there.
https://ceniehoff.com/Documents/Ctrl_Hyperlink/TG18_uid252021124312.pdf
There are 2 screw terminals ON THE REGULATOR. The first question I always ask, is with the engine running, what voltage do you see on the forward-most ENG terminal(closest to the pulley). It should be 24V. It must be 24V to enable the regulator to start making power. If it is there, what are you seeing on the output terminals? The alternator needs to see a valid 12 out of the middle of a 24v battery relationship on it’s output terminals in order to function properly. So bad or mis-connected batteries can cause the alt to not perform properly.
If 24V is not present at the ENG terminal on the regulator, with the engine running, You should still have 12 and 24 on the alternator output terminals from the battery, so if the forward regulator terminal doesn’t have 24, use a test wire to jumper battery 24V from the alternator 24V output terminal, and see what that does for your output voltages. Working properly you should see 14.1/28.2(With proper batteries and connections). If this works, then the alt is OK And you will have to troubleshoot the alternator excite control circuit…
As for upgrades you can fit a 200A dual volt from a humvee, or you can put in a straight commercial 24V alt and derive the 12V using a 100A bussman or vanner equalizer/converter. Not sure what the 200A humvee dual volts are going for these days, but the 24V alt and converter route can be done for less than a replacement 1506 alternator…