My NL M673 generator has 2 alternators.
The main alternator
generating 110V or 230 V AC which is of no concern in your case
and a small alternator with built in rectifying diodes and regulator which supply some 14 V DC to charge the generator' s starter battery.
When the generator is running , and nothing else charges the starter battery, test the voltage on the starter battery which may be more easily accessible than the innards of the generator:
If you find some 14 V: Then your alternator is effectively charging.
If you find the same voltage as when the generator is not running, say 12.3 vollts, then your alternator is not charging.
Before you despair, do creep down below and make sure that the regulator is Inside the alternator and not outside. Test the output voltage on the connection points of the alternator itself.
If you find zero volts, gen running, the regulator, and possibly the rectifying diodes are dead.
If you find 14 V good news: Your alternator is OK but the connection to the starter battery is severed somewhere , most probably by a burnt fuse.
For my NL gen, I found spare parts
at a competitive price
They also mailed NL wiring
diagrams to me but, although signedby NL, they contained substantial wiring