Cannot offer much here except:
Do you understand simple electrical switch circuits in general?
An electrical interlock is almost always wired as "normally closed".
Meaning, think of a light-switch. When it is on, the switch is said to be "closed", so the current flows.
An interlock circuit would usually be wired so that the current flows through the interlock switch normally (when the door is closed and locked). The circuit being "on" says "all is OK".
Opening the emergency-door turns the interlock switch "off".
(this is common in many things other than buses, like interlock circuits in industrial control circuits for manufacturing processes)
This is so because an interlock by nature is something important:
In the case of an emergency door or window on a bus, you don't want the bus to run if the door is open. So the interlock switch is probably wired so that the interlock switch on the door is "ON".
In that way, if the wires running to the switch were ever broken, the operator of the bus would know about it and act to find and correct the broken wire.
If it were not done this way, the wires running to the interlock switch could become broken somewhere over time, and the operator would never know until the time the interlock was needed (to stop the bus running if the door were open). And then it would not work to stop the engine running... nothing would happen.
It's a safe bet that the circuit to your emergency-door interlock is wired this way.
Why? Because when you removed the door, you broke the circuit. And now the engine won't run.
Assuming this is so, what you must do is locate the ends of the wires where they connected to the interlock switch on the door, and connect the two wires together. (then apply electrical tape to the spliced connection to insulate them)
And if there is also an emergency window, do the same there.
Then try to start the engine.
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eta:
Having typed in all the above, then I searched google and found:
https://www.skoolie.net/forums/f10/how-to-remove-the-thomas-vandalock-ignition-interlock-10956.html
