I believe Boyer-Bransden do a 6V kit but I haven't tried it. I do have a 12V Boyer kit on my Trials Cub and it's absolutely fine as long as you have a decent battery or a good alternator.
My Cub had no battery and I discovered the hard way that if the voltage to the ignition is too low, the Boyer kit magically sets itself to full advance. It's not fun for starting, but they claim to have fixed it in their MkIV version.
Personally, I'm changing my B40 (just started the rebuild) to 12V, which is a pretty easy job. In fact Boyer do a combined regulator/rectifier and ignition kit for just that purpose.
Another alternative is the Electrex system, which costs bit more, but replaces the alternator and ignition system completely, thus doing away with the points etc.