I don't think you will get much larger of a capacity battery to fit in the stock tray
What people do is strap on an extra battery after the charging failure occurs
A dash mounted volt meter is a must have if you take trips on your bike
The trick is to know as soon as possible that you are having charging problems so you can act accordingly
like shut off the main beam and park where you can push start and figure out how much power it's going to take to get you home