I would disconnect the battery and take it out of the bike. Then I would put a battery charger on it (2 amp max) for an hour or so. I would then add water to the full line (wear safety glasses). Then I would install the battery and use my $10 digital voltmeter to see if the charging system is charging the battery.
The battery should read 12 volts before starting, and about 13.5 volts at 5,000 rpm. If it's still reading 12 volts at 7,000 or so, the alternator, regulator, rectifier, wiring, or battery is bad. Then you need a mechanic or service manual to do more troubleshooting.