> How many aftermarket manufacturers will even pick up the phone for a one-off custom part, spec or no?

That would depend entirely upon the part.

With (detailed) specs being available, quite a lot of skilled repair techs would be able to bodge something competent together (!) to meet those specs.

Re-manufacturing completely 100% good parts could be a thing sure, for sufficient volumes.

But real world situations for low volumes tend to be "lets get it working well enough using the tools we have available". Creativity is required. ;)

The results for those situations are improved dramatically if there's detailed, good quality docs easily accessible to the people working on it.