That's a really good question, I know that electric motors will act as generator rather naturally, but the regulation of the voltage is the thing I think you will have to solve unless there is a ready-made kit.
I mostly know electrics, and I know my nephew's razor electric pocket bike would work as a gene, but it's designed to run on 24V, so I don't know how that correlates to what voltage it would put out as a gene, but I bet there is a pretty simple off the shelf voltage regulator that would make it happen. I'll ask the guru's at ES and see what I can find out, I am curious too as I would want to do this with my first gas bike.