You need vacuum to shut off the engine which can be accommodated with an electric solenoid or a Diesel key switch.

You need a glow relay which takes +12V when the key is in run, +12V when the key is in start (tap off the neutral start switch) and ground and sends +12V to the dash glow indicator. Later style afterglow relays have a connection to a coolant temp sensor either in the head or beside the thermostat.

The 602 engine has a vacuum pump for the brake booster, engine shutoff and ACC.

ALDA takes a pressure signal from the intake manifold with an overboost protection solenoid that requires +12V when the engine's running.

Flying saucer can be bypassed. Route the forward port of the VCV through a damper to the tranmission modulator. You might have to back off the modulator a notch or two to soften the shifts.

There's a computer for idle control but IIRC you can bump up base idle to where it'll stay running on its own.

You might have to bypass the Klima relay so the gasser ACC will engage the AC compressor.

Sixto
87 300D