larry moe 'n curly says...

Any luck finding a substitute in a power supply, such as
one for a PC? 500V MOSFETs are common in them.

I think you have to use a 500V transistor for the 12V
flash for the same reason vehicle ignition systems use
transistors rated for hundreds of volts on the 12V side
of the ignition coil -- big voltage spikes.

You could practice soldering the flexible circuit boards
of junked PC keyboards. I think they're made of mylar
polyester, while that brown flexible plastic in your
camera is probably kapton, a plastic with a much higher
melting point.

Thanks for the suggestions. I succeeded in desoldering the
Gate lead of the transistor so I could test it
out-of-circuit. But, it tested ok. So that wasn't the
problem after all.

However, after reassembly and doing a global reset of the
camera settings, it's working properly now.