[EC900]

A cheap Harbor Fright multimeter will suffice as 12.3v won’t even start a motorcycle.

Was there any slow or hesitant turning over, and then start up… if so then the battery is too weak and unable to hold a charge - even if using a battery tender.
Generally if the problem is intermittent then likely a ground connection. An faulty alternator/voltage reg that won’t charge a running engine and once battery is discharged, will stall the engine.

So I agree with other that the battery age and the main ground connections (often neglected) as being culprits, but at normal idle, the output should reach +/- 13.5 to 14.5 volts.

Instances when removing fan p, it can cause the ground wire there to break on reassembly or just from age as they turn brittle.