Is the battery good under load? Put a multimeter on the battery terminals, keep them there. While hitting the starter to put some load on the battery (easier with a helping set of hands), check the multimeter. If the battery sinks to ~10V while under load, it's in need of replacing.
Even with a battery that's dead enough to not show 10V while cranking and is unable to start the bike, the bike will still run if you can jump start it from another battery. Been there done that. Just don't let it stall, and hope your carbs are clean enough that it will idle without stalling.
Since we're talking about jumping the battery, remember to never jump the motorcycle from a running car. Just the car battery (non-running car) is fine. However, the alternator on an idling car puts out too much voltage and can fry the hardware in charge of regulating and smoothing the voltage going into the ninja's battery. No bueno.
