It's a common problem when custom building.
There are plenty of great old parts out there if you know what you can use and where to look.
Not always do they come in "plug and play" state. To ensure a fine end build they must be thoroughly checked and rebuilt with all the consumables replaced: Oil, Seals, O rings, Gaskets and the like. Most often I must even make them a little bit better than new. You see mass production gives us parts that are affordable most of the time, but things are often rushed to save time and - as we know know - time is money. So when we are custom building this is where the magic happens - we invest time to make things shine.
I have a build on right now - codename "Chappie" after the film about the smart robot.
It's a high front end frame - already got a great stance. It needs forks a certain length and many bikes have forks a lot shorter these days. In order to have great front end stiffness I opted for USD or upside down forks and off road forks are made to take some stresses road bikes never get. So I did some looking around and chose WP forks - originally Austrian high end made for small run off road bikes - I found some KTM units imported from California in very good condition from a 1994 KTM EXC 400 Factory Racing model needing a small bit of love.
Parts for things 27 years old I seem to forget are not that common these days (part of growing old is living in the past!) - luckily I have some excellent contacts who can find things from who knows where. Seals and bushes where found and ordered - O rings where a little harder to find now that some OEM parts are no longer available - but if you know where to look they are available and of higher specification than factory.
I am adding all the parts to an XL sheet so the new owner won't be bleeding from their eyes at 3 AM like I was :-)
Anyway back to making mass produced parts better. The lower fork castings that hold the axle/spindle and screw onto the lower fork tubes - well, they cast these things by the thousand and casting means pouring molten metal into moulds that split in half - this process leaves a "flashing" line on the casting and it certainly does not add to a slick finish. So when cleaning up these parts for coating I took out the trusty Dremel and ground away these ugly lines. I noticed also that as the two castings were different shapes (one side has brake caliper mounts) they were needing a little machining - I intend to make some fork tube guards - and need a flat fixing surface. So I fired up the Milling Machine and levelled those surfaces back. Yeah that's 5 hours of time the factory does not have and the results are worth the time invested.
You can hop over to my Instagram and check out more detail.
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