This is an electromagnet built out of high loss soft iron alloy material that if it's having a really good day might reach tens of volts per millisecond voltage rise rates when the circuit is opened nothing like a ferrite core inductor in a SMPS that could have KV's/uS voltage spikes.
Any half assed power diode with a several tens of amps current rating would be more than sufficient for the free wheeling circuit.
Believe me compared to modern solid state power supply components these things have the electrical properties of steam engines not high end sports cars.
They may have gobs of power but they are slower than hell when it comes to changes in speed.