I am such a person.What would be the motivation for doing this?.This so called '6V' output is in fact a weird looking ac.output consisting of short ac pulses of unequal pos. and neg. excursions, with no output in the duration between them.These pulses are of variable amplitude (roughly proportional to engine speed) .The only reason they are called 6V by the uninitiated is that you can light up a 6V incandescent light bulb with it.If you want to try to charge a 12V battery,I can give you some circuits that could work,but first give me an idea what you are about.Do you have a multi meter and some passing acquaintance with things electrical?Trying to run constant voltage dc. devices like LED's directly from that kind of ac. source is risky,some LED's can't stand much reverse voltage,also once you exceed the firing voltage (around 3.5 V typically) the current increases quite rapidly,so you stand a good chance of either stalling your engine or blowing up the LED.it's better to store energy by charging a battery,this also can give you a 6 or 12V source instead of that messy varying ac.output.I very much doubt that a secondary coil can be implemented,it has to conform to the radius of the flywheel VERY closely (within 1/32") , then you have to find the space to put it,it also has to match the circumferential dimensions of the flywheel magnet in order to close the magnetic circuit and get any useful output