FAA Advisory Circular
If you are flying a vintage aircraft, experimental or not, you must take note that the US-FAA has published AC 23-27 which details guidance to both owner and or restorers and FAA aviation safety inspectors when collecting information needed for an FAA approval when parts or materials used in the original construction of the type-certificated airplane are no longer available, or newer, more appropriate materials are now common and would be more appropriate to be used when repairing or replacing components.
FAA Advisory Circular AC-23-27
Excerpt from the document:
This AC provides guidance for substantiating parts or materials to maintain the safety of old or out-of-production GA aircraft, or other GA aircraft where the parts or materials are either difficult or impossible to obtain.
This AC also provides guidance about the data required to gain FAA approval for making these substitutions.
To continue reading see the pdf below: