A lot of “replacement” parts that are compatible with older bikes are being produced by offshore third parties. It is not too unlike parts for older cars. You might not be able to get an OEM water pump, but you’ll get one that works.

The biggest issue with vintage is the value is in the vintage components. Unless you are looking at specific highly sought after bikes, stripping lightly used parts off a frame and selling them individually will be more profitable. This also translates to buying them will be more expensive.

John