If the battery remains fully charged when disconnected then it seems the battery is OK and you must have a drain.
I have a recollection that the digital speedo can cause a battery drain problem although not as extreme as your situation. You could try unplugging the speedo and seeing if that makes a difference. Please let us know!
I am sorry but I don't know what other gear lever might fit.
I am not sure on the gear lever compatibility someone else may be able to help with that one.
You could possibly be looking at a duff battery, I don't think there is anything to drain the battery when the ignition is off. Maybe try charging battery and leaving it off the bike for a day or two then check the voltage?