I think another Zero motor, Sevcon controller, and 250cc+ motorcycle engine (essentially regenerating) would be the best way to do this for weight and compact design. The only generators that produce 6kw&+ that weigh little enough to be pulled by a Zero without increasing demand are crazy expensive and in use by the military to power UAVs. turbodiesel and high-altitude air-cooled. 3 Honda EU2200's would also approach this.
While it would be interesting to see, there are far better ways to make a hybrid bike and the equivalent mpg this would result in (40-60mpg) is kind of atrocious for the level of work involved.