Depending on how the machine is wired the likely causes are the ignition switch or the Rectifier/Regulator. does it charge when running? If the voltage running is not higher than battery at rest, it's likely them RR. Could be that even if it charges, but more likely if not.
A meter in series between the battery post and ground will tell you if it is a parasitic drain. Repeat between the Reg's + lead and ground, then ignition switch and ground to isolate.