Try
www.yuasabatteries.com
for what might fit your bike. A "bigger" for physical or electrical capacity is my question.
The charging systems on motorcycles are designed with a balance in mind. Sometime you can get away with a larger alternator/generator without damage(burned wiring, instrument lights failing,short life of headlight bulbs).
The battery box or where the battery will go limits what size you can replace it with. Battery technology has made advancements in cell size and amp output so if you can put a new battery of the same size as your old one it might have more capability to start the bike.
Are you having problems with starting? If so clean all electrical terminals (handle bar switches,electrical plugs, bullet connectors,battery terminals and where the ground cable connects to the engine/frame) install the proper heat range spark plug with the correct electrode gap. Valve clearance must be checked as this is usually the first thing not looked at and fingers are pointed at the coil(s) or carbs or bad fuel.
