I use to have CD-200 long time back, and got it customized too but sadly my father insisted to sell it and get a car.
I had the same problem so I got it altered to 12 volts, They do little alteration to the wiring, change al bulbs, the main generator and it will run better.
The Guys I use to buy my imported YUASA 6V battery has no more as there is no demand, but you can always do a Jugaar and install a 6V VR dry battery made for different toys/ rechargeable fans/ lights etc.