* "Fuel distributor piston spring"! What are you referring to? There's no spring on a K-Jetronic plunger (what you're calling a piston?). Or are you talking about the spring in the line pressure regulator? That, along with some shims, is probably available in a pressure regulator kit from MB.
* Disassembly of the fuel distributor (splitting the halves), if that's what you're doing, isn't particularly a good idea. The major parts are lapped in and precisely assembled, something you're unlikely to duplicate on the bench. Better to run a strong cleaner through it. Most likely whatever symptom you're trying to cure is caused by something else anyway. The fuel distributor isn't a very common replacement item (yet it gets blamed for a multitude of sins).