At the very least, if you want the matching fork in the right size, it will be hard to find and wind up costing as much as the "cheap" frame. Don't ask how I know.
Not surprising! I bet a specific ancient fork of your make and frame design, in decent condition, is more rare than the hens' tooth vintage frames that you show! Plus, it means somebody else was dumb enough to let his/her fork get separated from his/her matching frame.
OTOH, in a crash one could end up with nasty creased main tubes and a decently well-surviving fork, so ....
I engaged in fork experimentation on my 1984 Trek 610. First I thought the steering was unstable when climbing at low speed, so I though more trail would be the solution. Off to the frame shop, cold-set it backwards about 10 mm, and rode. A sport-tourer was transformed into having the steering of more of a road racer (59 mm trail), but still not so good in a slow climb. All my climbs are slow climbs BTW. I think the improvement was mainly due to the whole frame having been aligned very precisely, especially in terms of coplanarity, and I had a brand-new Campy headset installed. Natural fork response was as good as it can be. I also inherited a problem of toe-wheel interference even without the fender I now wanted to add. Frame shop #1 did not want to cold set my fork blades again, he was concerned about weakening the CrMo, possibly 531 blades.
I discovered a (now-closed) local shop, Ypsilanti Cycles, where the owner has a frame building background and he was willing to design and build a low-trail high-offset for for me with the ride height equalized (head tube angle as specified by Trek). He could not paint it properly, but he did rattle-can it pretty darn well! Now I have offset about 65 mm, 73 head angle, and trail about 38 mm. The for blades are pretty long and I handled the resulting brake reach with a Mafac Racer, about 70 mm. But to design that fork was essentially to measure up the bike, model it in BikeCad, then model a new fork which made my BICYCLE have desirable properties, which we discussed, along with my riding preferences and habits. The bike ever since has climbed in a straight line way better than in the past.
I think the design of the custom fork was nearly as much work as designing a custom bike!
For me: Don't lose your fork!