Well, like I say, most of its flying now would be low, where you wouldn't normally use the turbo. My original concern was basically that it might be hard to have a 100% working warbird if you couldn't maintain the turbo. As you see in the pics I posted, it's a not insignificant part of the aircraft. I guess even more so for the P38 since it is a visible part of the airframe.
It seems the answer to my question really is that if the turbo isn't working, they just don't use it.