Editing Client

I have made and managed many Private Servers before, but had never touched on client editing.
Every time I try to decompile it, the code gets messed up, even with AS3 Sorcerer.
Do you have to build everything from the bottom up, or is there a way to decompile it safely?

Needs a lot of time fixing and tweaking to get it 100%

Then how do you fix the code?
That is what I need, not some obvious statement :v