I don't have one, never seen one.
It seems you have ruled out battery, I've assumed you went there first.
If the machine is years old but the battery is new
I'd start with
loose cable connections or oxidised rotting/corroding main cable cores, usually nearer the battery end of the cable.
Commutator, contact brushes, voltage regulator, dust infiltration, bearings, sintered alloy bushes on the actual alternator section of the starter/alternator, and also check the battery age/condition/quality/electrolyte levels.
(and it doesn't mean you can fault find with the first 'have a look')
Also lead acid DC batteries with swollen sides, tops/bottoms are a reasonable indicator of mechanical collapse of the plates/paste/packers.