Dont even need a key....
All you need is four wires from the engine to the CDI, blue and green from the spark advancer/pulse gen, magneto output from engine, and a ground to the CDI, then the coil wire from the CDI to the coil, if you want a simple kill system wire a toggle switch from the ignition wire to ground on the CDI, switch on and grounded its killed. The two yellow wires from the engine are the alternator output, you can run simple lights off it, up to about 5 amps but its AC voltage, so you need to regulate it, you can rob the stock rectifier/regulator off the frame to use that.
All you need then is pull start and your good to go. Normally i would say don't do it to as they are a rarer beast, but hey if you do make it cool! If you want electric start thats pretty easy, just pull the starter solenoid from the frame, simple deal all you need is a button switch to energize the solenoid, battery cable to solenoid, and a cable from the solenoid to the starter. You can even charge the battery off the engine along with lighting as i mentioned.
I have seen something similar to what you want to do, the owner of it built a nice control panel with toggle switches for the kill switch and lights, push button for start. Only limited by your imagination. The 200ES motor being a shaft drive would be the perfect motor for that. If the frame is shot might as well give it some life elsewhere.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....