he engine shut down device is a vacuum powered bellows inside the
injection pump. Insufficient vacuum due to a leak - somewhere - or a
broken shut down, is the problem. Leaks can occur in the door locks or
climate control. Try locking the doors when you drive it so vacuum
builds as its driven, sometimes this works.
Meanwhile, to stop the engine push the STOP lever on the throttle
linkage that's bolted onto the engine and it WILL stop!