For a 1979-era door, we're usually not swapping in anything exotic. Torsion springs sized for a standard double or single door, steel rollers, standard-width track sections, and the usual opener drive parts - these are stock items we carry on the truck. Where it gets slower is anything specific to an original wood or early steel panel door that hasn't been replaced since the house was built. If your door is the original factory door and a panel's cracked or the hardware mount is bent, we may need to order rather than fix same-visit.
We're honest about that limitation: if a part is genuinely out of production, replacement of the whole door is usually cheaper and faster than hunting down a match, and we'll tell you that straight instead of stringing out a repair.