Major part of any restoration effort is the careful and meticulous repair and reconditioning of various assemblies and parts to their original working condition.
Compared to modern vehicles, which have readily available parts supply, finding the right replacement parts isn’t so straight forward and needs a lot of research online, establishing and using personal contacts,
Its not uncommon for all efforts to lead to a dead-end, then the only way out is to use engineering knowledge and skills to recondition the old parts and in many cases go through a complete reproduction, either in house, or using various engineering outsource suppliers (laser cutting, CNC, precision machining, 3d Printing, casting)

Here are just some overhaul projects:

Mercedes 280CE 1983
The CIS mechanical injection needed an overhaul, when its dosing piston had a sticking motion and fuelling was erratic, at best.
While membranes are still available in rebuild kits, it was more economical and faster to model the original part in CREO and use water-jet cutting to make several pieces locally.

Piaggio Vespa PX 50 XL 1986