Originally Posted by
Konstantin_L
Yes! I have just independently realised that the thread pitch is wrong, so I cannot use the fork regardless of the crown race.
It seems in lieu of a Peugeot fork appearing on eBay, a new frame / whole bike for me

Wait, no -- it sounds like you've determined that the proportion of the fork is correct for the frame, so I think you just need an English threaded headset with a 27.0 crown race. I think you can just get a cheap Tange Passage and it would get you going.
Link:
https://www.benscycle.com/tange-seik...saAs4zEALw_wcB