If this is the system I'm thinking of, there should be a rectifier, probably mounted on or near the sheetmetal cowling of the engine.
There will be 3 wires attached to it. Two of the wires will come from the alternator coils under the flywheel. The third wire will go to the battery.
Look the wires over, be sure they are all connected and in good condition. The wire to the battery should show battery voltage or test hot with a test light. It may be wired through the ign switch, try it with the switch on.
The rectifier can be tested with an ohm meter. With the connector unplugged, test between the battery terminal and one of the alternator terminals. It tests like a diode, which it is. With the leads connected one way, it will read some continuity, reverse the leads and it should read open. Do the same for the other lead, checking between the battery and the alternator terminals.
The problem will most likely be in the wiring or rectifier. The alternator winding rarely fail unless it's been physically damaged.
Hope this helps.