Those who can, repair. Those who can't, simulate.
For almost all vintage computers software simulations exist. The „SimH“ suite is well known for classic mainframes and minis.
On the other side, old discrete computers like DEC UNIBUS/QBUS PDP-11s must often be repaired. Then we replace defective parts sometimes with ancient or modern hardware.
A special case are disk drives, which are most sensitive and always likely to fail. SD card solutions exist for many systems like C64, Amiga, Apples or SCSI.
So today a vintage computer can be a mix of software simulations, recreated hardware and original parts.
Uncountable repair and simulation projects exist for DEC PDP-8s and -11s, making these machines the perfect model case for "restoration by simulation".
See several PDP-11 projects arranged on a scale for their „virtuality“ or „physicality“ (are these even english words?)
Project Link collection:
DECBox, PDP-11 in a VT100 terminal
BlinkenBone for reanimating console panels
PDP2011, Sytse van Slootens FPGA recreation with PiDP11 interface
PiDP11, Oscars Vermeulens famous PDP-11/70 replica
SCSI2SD, SCSI "disk drive" based on SDcards
Well, sometimes we have that strange part mix, with repair parts fetched from dead PDPs, assembly often done in a dark night in a sinister lab, and life was given by toggeling the big power switch ... See here one of these in action: