Another tinkering option is DewCatTEA Bob's brilliance! Check out his diagram at the bottom of the post! In this post http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=394&hilit=12+volt+conversion+scrambler&start=20 Bob explains how to modify an existing Ducati 6v system to harness the wasted power using bridge blocks and convert to 12v
"As far as the difference between 6 & 12 volt charging goes, there would only be a slight difference in the RPM needed to actually begin any actual charging of the battery...
As a 6-volt battery's std.volatage-level would be able to feel charging-pokes from the alternator's voltage-peaks, at a slightly lower RPM than a 12-volt battery would,, as the RPMs climb, from a non-charging to a charging amount of alt.power-output.
So while that may seem like a worthwhile-bit of an advantage for prefering to go-with the 6-volt choice, it's not really very critical for normal (non-constant low-RPM) riding. _ Because a 12-volt battery won't begin to act as a load (and begin consuming alt.power-juice) at the same lower RPM, thus allowing the alt.power to fairly quickly build-up from the 6v to the 12v level and therefore tends to reduce the otherwise seemingly quite noteworthy advantage, (which is actually only a slight one). _ (Keep in mind that a battery that's able to receive any CHARGING-juice is also a 'load' that's partially wasting the charging-system's power-juice, since unlike a capacitor, the battery can't give back everything it has taken !)
__ And from the other point of view, a 12-volt battery system will not as likely make any use of a voltage-regulator."
The AC-output (of that lighting power-coil's two wire-leads) would then need to be full-wave rectified by a 10 or (25-amp) bridge-block, of which it's DC-output should then be switched so that it's only turned-on when the lights are running off the battery.
____ The other alt.stator power-coil -(meant for ignition & brake-light), would also have to have it's Yellow & White wire-leads connected to another separate bride-block, so that it's full-wave DC-output could then keep the battery charged and run a 12-volt battery-powered ignition-system. ____ The full-wave diode-bridge for the lighting alt.coil should have at least a 6-amp rating, and the battery charging circuit's diode-bridge should be at least 4-amp, and both ought to have greater than a 50PIV rating.
__ But those bridge-rectifier units which you may find, (while quite cheap), may be difficult to mount & wire-up.
So I'd recommend buying a pair of bridge-blocks with 25-amp & 200PIV, as that is such a commonly produced diode-bridge size, that it's quite cheap for it's specs and also easy to mount & wire-up !"