I have a .380 Walther PPK with a cracked slide...the result of year of firing by its ex-owner. The only potential slide replacement I found thus far is coming from a .32 pistol. Is the slide compatible with the bigger caliber ? 

Thank you 

No.

The 32 barrel is smaller in diameter, and the .32 cartridge is also smaller in diameter, hence the breech face and extractor will be a problem with the .380 cartridge.

I've never seen a cracked PPK slide.  I wonder if a lot of that silly, so called +P Buffalo Bore ammo went down the pipe?

You will need a .380 slide, no way around that.

Look up M&M Gunsmithing, the guy is the go to for Walther PP series pistol work, perhaps he can help you out.

Why don't you contact Walther I'm sure they have replacement slides available for sale.

479-242-8500 Ext 7.

I would not count on that, as it will depend on what vintage the OP's PPK is.

If it is one of the Smith and Wesson made late PPK/s models then yeah, they might have one.

If it's an Interarms made or earlier piece, no way.

I'm still curious about the crack.  Usually what you see cracked on the PP series pistols is the safety drum, this comes from too much de-cocking without lowering the hammer down with your fingers after turning the decocking/safety lever.

Numrich is worth a try, again like mentioned previously. Make sure you know which version you have ; Mannhurian, Interarms, S&W etc...