There are 2 airworthy Lancasters, BBMF own 1 and the Canadian Warplane Heritage Museum own another. Not been more or less than those 2 for many years. We may have another one at some point with Just Jane being slowly restored to flight, although she can taxi already. BBMF one has been airworthy since 1967 and the CWHM one since 1988. No crashes.
If you mean the B17 crash in Texas, that was a mid air collision where a P63 collided with it. They apparently weren't told to keep to an certain altitude to keep separation.
Once a part cannot be repaired or replaced as new, and has no equivalent available. It's not airworthy. It's very simple.
It's genuinely surprising how little is original on a lot of of these restored warbirds. In some cases, it's just the data plate and the rest of the wreck is just kept 'somewhere'. Although I visited one restorer who worked meticulously to use as much original as possible. His FW190 was over 95% original metal, even the bulb in the gunsight was the original one.