Do you mean mount a sprocket on the front? Use a sprocket mounting kit that mounts a sprocket for on the back on the front. I guess. To stop the chain from spinning the motor when the motor is not on, there is a sprague type sprocket thing to let a motor free wheel I can 't tell you where to get one but a pocket bike I had for awhile had the one. With the engine off the drive sprocket would free wheel like a bicycles.
Norman

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norms-iron-works.tripod.com
motor assisted bicycle engine headlights that work off of your white power wire. check it out