Bandit 400 LTD: Mysterious Ailment

It’s been sitting neglected for quite a while now.
 

 
So it runs, then the engine dies, runs, then the engine dies.
Or so I’m told.
The prime suspects were the rusty tank and the fuel petcock area.
Fuel petcock: OK
Tank breather: OK. So that’s not it.
When I measured the charging voltage, it was 13V, then dropping to 12.9–12.1V and continuing to fall.
It’s either the rectifier and regulator or the stator coil.
Since it barely hits 13V without revving and is charging, maybe it was okay when running slowly and steadily.
Now that I know the direction to look, let me pin down the exact cause.
 

 
The speedometer gear is broken.
Results
↓
Stator coil: OK
Regulator rectifier: NG
Bingo!