I think it would take a lot of work to reprogram the computers and key systems to the aircraft. I was an avionics tech (AT) on FA-18 and the aircraft had two flight computers, two mission computers and a central computer for all communications and displays and all the MUX buses that are needed between them. Keeping those in working order isn’t that easy. Even when you have a dozen in the supply system that are “in working order”.
I found that the man-hours pre flight hour was a joke. We basically made sure that we each had our 12 hours accounted for in paper work. Also half the systems a civilian would not need and could not get, Weapon systems, release and control checks, ECM. T-FLIR. Radar. IFF…
I can’t even comprehend that someone would be able to afford the cost of keeping a modern jet in the air. But I can’t see how someone can afford keeping a WW2 fighter in the air.