Just trying to think this through with you. I'm no expert but have had similar experiences on other bikes.
1. Battery check. Charge the battery overnight. Then measure battery voltage across the terminals with a multimeter with no load , then switch on ignition and measure, then measure whilst you crank engine, then measure whilst running at 2k revs. Post all four results on here afterwards.
2. Stator output check. Put the ground (black coloured) probe of your multimeter, now set to measure resistance, to any ground source (clean metal body work or earthing point). Attach the positive probe to each of the terminals in the stator's electrical connector individually (there are three to check). The reading should show Overloaded or OL. If any of the readings shows any resistance, the stator has failed and must be replaced.
3. Rectifier check - To check the rectifier, connect the meter leads to the bike's battery as it's running. The reading should not be higher than 14.5 volts or lower than 13.5 volts. If the reading is higher, this means the battery is overcharged and you may need to replace the rectifier.
4. If there are connectors between the stator and the regulator, check that the wires to the connectors are soldered and not press fitted, big charging losses with loose wires to the connectors and even a potential fire hazard.
5. Is there any load on the battery when the ignition is switched off at all? Sat Nav, alarm etc?
6. Check that the battery terminals are cleaned on both surfaces and nice and tight - we've all done it.