I assumed you have checked the bulbs?
the neutral and oil connections are both near the output sprocket and as such can be brittle thus break internally if disturbed.
I'd earth the leads and see if you get light. then check connection/fuse for power.
ask the seller what ampage it charges at. Its obviously not an optimate, which will monitor the battery, but for a 6v battery.
why don't you just buy a new battery? can't be that much!