Hi guys, the Japanese call all alternators dynamos. The difference in the small tractor units is that often the rectifiers (diodes) are mounted in and use the regulator housing as a heat sink. The AC sine wave out of the dynamo is 24v peak to peak. The dc (rectified) voltage out of the diodes and on to the battery should be between 13.8 and 14.8 volts. Unless the dynamo has permanent magnets, check to be sure that exciter (field) current is being fed to dynamo with ignition on.
Also check field draw to be sure brushes are ok. The fact that you problem is sporadic could indicate poor brush to rotor tracking.
lots of luck,
george