$0.93 battery charging circuit for the Huasheng 50cc motor and a 12V lead acid battery:
Rectifying Diode, 1 Amp
http://mouser.com/Search/ProductDetail.aspx?R=1N4935-E3/54virtualkey61370000virtualkey625-1N4935-E3
Resistor, 750 ohm, 5 Watt
http://mouser.com/Search/ProductDetail.aspx?R=286-750-RCvirtualkey21980000virtualkey286-750-RC
Zener Diode, 14V, 5 W
http://mouser.com/Search/ProductDetail.aspx?R=1N5351BGvirtualkey58410000virtualkey863-1N5351BG
Connect the Killswitch wire to the Diode with the white stripe AWAY from the Killswitch wire. You can still use the Killswitch.
Connect the white stripe side of the diode to the Resistor (either side of resistor).
Connect the other side of the resistor to the Zener Diode white stripe side.
Connect the other side of the Zener Diode to the motor body (ground).
Connect the battery positive between the Resistor and Zener Diode.
Connect the battery negative to motor body (ground).
Headlight, taillights and other loads are connected as usual to the battery with a fuse in line.
This circuit has not been tested, use at your own risk. I believe you might see a small charging current in the range of 0.1 Amp. This is not much but if you ride mostly during the day and only occasionally at night a small trickle charge might be all you need.
I suggest buying additional resistors, perhaps 400 ohm, 600 ohm and 800 ohm. If the motor will start and run with a 400 ohm resistor, I would use it because it will allow more voltage to the battery. The absolute minimum size of resistor to use is 100 ohms. This minimum is needed to protect the Zener Diode. I am not sure if the motor will run or be shut off by this low resistance, it depends on how much other resistance is in the circuit. I used a potentiometer and decreased the resistance between the Killswitch wire and ground until the motor quit. The motor quit at 600 ohms. However, the charging circuit Resistor is on the other side of the Rectifying Diode from the motor, so the motor may get enough current during the time it is producing less than the 12V it takes to match the battery voltage. I would experiment. You are not going to hurt anything as long as the resistor is over 100 ohm, it is just that the motor may not start.
You may want to put a switch in line between the Killswitch wire and this circuit. It may be easier to start the motor when no amps are being drawn by this circuit. You could try it without a switch first. I believe it will work without a switch.
I have oversized the ratings of the components mainly because they are cheap and it makes the circuit more durable. Higher rated parts are also usually a little larger and easier for me to work with.
Theory:
The Rectifying Diode allows current to flow from the Killswitch wire to the battery but not in the other direction. When the motor is producing less than the battery voltage, no current will pass the Rectifying Diode in either direction. When the motor is producing more than the battery voltage (minus the voltage drop of the Rectifying Diode - 1.2V) then current will flow in the direction of the battery.
The Resistor does two things: it makes sure that the minimum resistance of the circuit is always above the motor shut off resistance which I measured on my motor. The Resistor also protects the Zener Diode from too much current.
The Zener Diode does not allow any flow of current if the voltage is less than 14 V. If the voltage is above 14V, the Zener Diode allows current through to the ground, "short circuiting" any current over 14V. This means that the voltage on the white stripe side of the Zener Diode will not exceed 14V. 14V was selected because it is a good charging voltage for a 12V lead acid battery. As long as the motor puts out over 14V, the battery will see 14V.