Old Mercedes W123 ingition key mechanism broke, so I made this handy universal replacement. You need to remove the inside mechanism and tumbler assembly from your key switch. Also remove the next part that disengages the steering lock.
This 'key' is tapered to disengage the steering lock as you insert it, push and turn until it engages into the slot at the far end that operates the electrical switch.
The spring for the 'start' part of the switch is quite stiff, and could break the little lug at the end of this key, so I recommend that you install a push button or toggle switch for the starter. I have one hidden under the dash, that I operate with my foot.
Remember that this will make the car quite easy to steal - if anyone wants to steal a 35 year old car, so I recommend a hidden switch for the fuel pump or coil as a further safety mechanism.