Hi,
I can´t find the attached schematic.
MOSFETs are selected by oltage, current, R_DS_ON, speed parameters ....
And for this distributors as well as the MOSFET manufacturers provide selection tools.
Use them.
Althow: "national competition", "sterialize hands", "12kV", "without much heat", "closed box" maybe are good side informations .... one still need to know the electric specifications to choose the right one.
71V spikes on Ltspice:
If you are not an experienced PCB layouter fro SMPS you will see a much higher spikes on the real circuit. On the other hand one may suppress them to a lower level with a suitable circuit.
Klaus