In the 1970s, MMC (Michigan Machine Co.) made a fully adjustable rear sight for the Walther PPK. The late gun writer Skeeter Skelton wrote about this novel sight and had one put on his favorite stainless steel .380 PPK.
Would this work? I don't know. MMC didn't make sights for long, and went out of business.
About seven years ago, I found an MMC adjustable sight for the Walther PPK on Fleabay. Cost me $50 when the bidding-smoke cleared. When I received it, it still bore a price label of $75! How's that for reverse inflation?
I still see MMC sights on Fleabay and Gunbroker.com once in a while, but they're most often for the 1911. If a Walther PPK adjustable sight would work on your gun, then go for it.
My MMC adjustable sight for the Walther PPK is very small. Takes an incredibly small Allen wrench to adjust it. It's nearly the same size as the original, fixed rear sight. It's basically made to find one good load, then adjust the sight to that load. Not so good for twiddling up and down at the range, though you could do it, I guess.
I adjust mine to practice with lead, 95 gr. cast bullets at about 850 fps. Then, I adjust it again for 88 gr. hollowpoints at about 1,000 fps as my carry load.
I must have one of the few Walther PPKs in the world to have a fully adjustable rear sight. I just wish I could find the rubber grips (Hogue? Pachmayr?) that Skelton also put on his PPK. No one makes a rubber grip for the PPK, though plenty are made for the PP and PPK/s, which share a common frame. The PPK's grip design is quite different.
Measure the rear sight dovetail in your pistol. Perhaps you can find a sight that will fit it, whose sight notch isn't much higher or lower than the original notch.
