Search for a 6 volt regulator rectifier.
On this link there is a hand drawn wiring diagram
Honda TL125 to CB125S Engine swap...... | Adventure Rider (advrider.com)
12 volt bicycle dynamo from eBay
bicycle dynamo generator 12v | eBay
The regulator rectifier will control the charge from the bike dynamo. Battery, I would use a small 6 volt rechargeable battery. The reg/rec suggested came from a Honda125 6v system. The 12 volt dynamo puts out enough power and through the regulator it should be good to keep a 6 volt battery charged.
Do I know what I am talking about, not only no, but **** no. But from my research this concoction should work. The smaller 6v dynamos won't kick out enough power to keep the battery charged. This is info my auto mechanic told me.
Right now I am still running with battery lights. Do not need this, but have researched it enough that I believe that it will work.