My memory of the DJ series is that you ave a 12 volt plunger solenoid that pushes against the fuel rack when there is no power to it, forcing the injection pump closed.
Pushing the preheat switch applied power to the glow plugs and the run relay which supplied 12 volts to the plunger solenoid, causing it to pull inwards removing pressure from the injection pump arm.
As the generator started oil pressure builds and the current for the coil of the run relay then comes through the oil pressure switch, the coolant temp switch, and a normally closed set of contacts on the stop relay.
When you wanted to stop the generator the stop switch energized the stop relay which opened the contacts carrying the current for the run relay which then dropped out.
The run relay dropping out interrupted the current flowing through the plunger solenoid causing the spring in it to push the plunger out, against the injection pump arm, shutting off fuel to the generator.
That's how I remember the circuit, but its honestly been 15 years since I had to work on one.