I want to edit an existing mod but I do not have the original PSC file for the script in the mod. I've tried using some Decompiler/Compiler apps I found but I don't think they work for some reason.

I tried Champollion and Pex Decomplier and Compiler apps from OneLessName on the nexus.

When using either one, if I decompile the PEX file and then compile it again the script no longer works in game. (even if I do not edit the file, just testing the decompile/compile)

Is there another way to decompile a PEX and get it back to a game usable state?

There's tools on the Nexus to partialy decompile script, but you'll need whole lot of experience to use it.

Actually there's no tool that completely decompiles .pex files into a useable format.