In the aircraft business, we are worried about planes falling out of the sky. These are aluminum tubes that are pressurized and depressurized on a regular, if not multi-times-daily, basis.
Our biggest concern isn't the engines, landing gear, interior, cockpit avionics, or stuff like that. All components can be replaced. It's the airframe itself that has fatigue life issues. Over time, aluminum decays. The constant stress of expanding and contracting can cause cracks, which are exasperated by further pressurization.
That being said, though, I have worked on military aircraft that FLY DAILY that were built in 1962 - that's well over 50 years of service. Of course they have been overhauled. There's probably as much patchwork as original structure. But they work. They are finally looking towards an EOL, but not yet. For an ordinary aircraft, it eventually becomes cheaper to replace than overhaul. For these extraordinary aircraft, not necessarily.
So, I don't see the P42's can't have their entire guts removed and replaced. No parts? I'm sure if someone knew they were going to refurbish a couple hundred, they would produce the parts. If the original tooling hasn't been scrapped, even more better.
As for the diesel engines themselves, you can continue to oversize bore the cylinders while increasing the size of the pistons and rings, but eventually, you start to burn more and more fuel, losing an economically sound efficiency. The motors, I would imagine, would eventually lose efficiency as well.
