For a door built in the 1968 era, the usual failure points are torsion springs sized for a heavier wood or early-steel panel, worn end bearings, and openers that have been swapped out once or twice already since the house was built. We stock standard torsion springs and the common residential opener drive types on the truck, because that covers most calls in a house this age.
What we don't carry as a standing item is anything custom - a wide carriage-house style door on oversized track, or hardware matched to a panel upgrade that isn't a standard size. In a town where income runs well above the area median, we see more of those upgraded doors than we do farther into the service area, and if a part is custom to that install, it has to be ordered rather than pulled off the truck. We'll tell you that on the first visit, not after you've waited for a callback.