If you are going to wind yourself a 12 volt coil, I urge you to reach out to Jon Pardue here on LiLHonda. He has done this before and he understands the science behind using thin gauge enameled wire here or thick for wrapping these charge coils - its a volts versus amps discussion here. I would not attempt to re-invent the wheel for winding coils when Jon already has experience with this.
Rick
I don't recall Jon ever mentioning having done this. Still...he knows his way around electrics and knowledge is power, in this instance electrical power.
That said, there's some DIY "failsafes" built into this. First off, while it is a volts vs amps balancing act, Ohm's Law sez that there's constant...watts. Volts x amps = watts, i.e. total electrical power. (Just to further illustrate, 1 horsepower = 746 watts.) Best guess is that a 2-pole (6v type) CT70 alternator can generate maybe 40W total output, max...regardless of volt + amperage
balance. There is, imho, an optimal balance, of those two parameters, which segues into the other "failsafe". Thinner wire should increase voltage...more turns; however, it'd be at the expense of amps. Thinner wire has higher impedance...a.k.a. ohms. It's also more fragile. OTOH, thicker wire being the opposite, would likely tend toward low voltages at lower rpm. Fortunately, Honda engineers did us a favor; they selected a wire gauge that has proved viable. Thus, I'd start there. Any further experimentation would be using wire diameters no more than 1-2 gauge numbers higher or lower.
Since we're already wa-ay out on this limb, I'd start with one gauge number lower (next larger wire diameter), winding the coil to the physical limits of the stator assembly. That extra metal (total amount of wire) should balance-out the impedance (and peak voltage) and provide maximum total output...watts.