It doesn't work like that Phlegethon. You can't simply take any game, blue print it and rebuild it. Some elements of the code are in a compiled format which can't be just be decompiled to a usable programming language. It is not a matter of encryption, but when code is compiled in a certain way (such as a .exe or .dll) it is converter to run in assembler code which is near on impossible to read. Even the most experienced programmers struggle with it. So in order to create new graphics DLL, you need the original sources to be able to edit them and then compile.
On top of these core dlls and the exe, there are then java classes (which we can decompile). They are compiled in a format which is then red by the Java Runtime Environment which translates these classes to assembler code which is then understood by the hardware. These classes only concern a few elements of the game engine and mostly encode game features (which enable the engine to use game content such as 3D models, maps, sound etc)
Really sorry mate, but it is clear from your responses you have no idea how software actually works. If it was as easy as you imply, then we would already have these update engine feature by now.