I had one just like it--sold 4 years ago. Really it isn't such a hard job to install a motorscooter or small deep cycle wheel chair battery. Just build a basket for it where the old battery used to sit. Batt costs between $25 and $50. Just put a charger on it at night. And you don't really need the battery--as long as you are spreading fertilizer --and don't need to spray.
I see the voltage regulator is missing. It doesn't really have a generator; it charges using magnets on the flywheel...well that is how it started out. I loved mine. Put about 1000 hours on it.
Retired now--don't need one.