parent tether. you can walk behind your kid while they ride and pull the tether to cut the engine. I supposed once they got the hang of it you could use a key ring or something to attach the tether to the child and if they roll it/fall off it will work the same way modern tethers do.

The guy I bought it from said that when he rebuilt the motor there was a factory restrictor in the intake and exhaust. he removed them and couldn't keep it from throwing him off the back so he put them back in.