I’m using a half wave rectifier. The full wave rectifier is normally the better way to go. But on these engines, the full wave gives you a weaker spark which can affect engine performance. Use a half wave rectifier and even a short in your lighting system will not affect the engine.
I have a bicycle 6v headlamp and an automotive 6v taillamp. I have a small 1.3ah lead acid battery. The resting voltage of the battery was 6.30v. I went for a 7 mile ride last night with the lights on in order to run the battery down. Right after the ride, the voltage was 5.89v. I checked the battery this morning and the resting voltage was 6.12v. I went for a 25 mile ride today with the lights turned off. The resting battery voltage was 6.24v right after the ride and settled at 6.20v about 2 hours later. It appears that the charging system is working but the output of the alternator is very small. I’m assuming that it is just a function of the design.
6.30v - initial volts
5.89v - after running lights
6.12v - after 12 hours rest
6.24v - after 25 mile ride
6.20v - after 2 hour rest
I included a zener regulator in my system but after seeing how weak the charging system is, I would guess that it is not needed. Lead acid batteries can stand a limited amount of overcharging better than most other battery chemistries. I will run my system with the regulator for awhile, but I will probably remove it later and test without it.
After this experiment, I would suggest a system using a single rectifier diode to a very small lead acid battery. And the most efficient 6v LED lighting system you can come up with. I'm going to look for an LED bulb for my tail/stop light next.
You don’t even need a wiring diagram. Just hook the diode (Radio Shack part no. 276-1141) up to the white wire. Ground the other end of the diode to the running engine. If the engine dies, reverse the diode. That determines the correct polarity so as not to affect the spark.
For negative ground:
----white wire----[diode I]----[+ battery -]-----ground
For positive ground:
----white wire----[I diode]----[- battery +]-----ground
Hook your lights up to the battery through a switch. You should be good to go.
Diode is about $1, 6v lead acid battery is about $14, switch is about $3, 6v lights can be whatever you come up with.