There are two approaches to electronic ignition. One is to connect the
coil across a power source, let the current build up then interupt it.
V = L di/dt so voltage rises and in both primary and secondary until
plug fires. That's how pointed ignition systems worked. Early
"transistorized" ignitions simply replaced the points with a transistor
triggered by points or optical or magnetic sensor.

The second is capacitor discharge. Here a cap is charged up from an
appropriate power supply and discharged through the coil primary. These
are generally better.

The power source can be a vehicle electrical system or a coil and
magnet.

Pretty quick and dirty explanation but there is just no way you're going
to do it with just a Zenner diode.

(I've designed and built a few of both kinds.)

Ted