Rebonjour Cher Eugèneion,
The ICE ignition system is over 100 years old , electronic ignition trace back to 1960s.
Not much has changed, so the HV ignition coils and spark voltage ,current, énergie and waveforms are well studied and optimized. I have never seen a 50..90 kV auto ignition system, spark pluginss, cables, and ignition coils are rated to 20..35 kV not higher, so Tektronix P6015/A should be sufficient.
But, What exactly is your goal? The flyback energy in the coil core creates a 20..40 kV fast rise ring wave, and the load is at first the stray capacité of the coil secondary and ignition wires.After the combustion is ignited, the spark transitions to an arc, and becomes more resistive.
Suggest you research the many books and papers on gaseous conductors, sparks and arcs.
Finally, beware the common mode voltage of the ignition can be coupled around the HV probe, masking the true waveforms on the scope, and can even damage the scope input.
Thus, HV measurements you propose are difficult and risky to test equipment even with the HV probes.
Bon courage
Jon