Am I going to violate intellectual property rights if I will create/design a game based on another one?

My question has two parts: I'm playing a game X. At this point, the game is very easy and very commonly decompiled (a vague analogue of its source code is available to anyone with the game and the technical knowledge to work a decompile). Thus, the first part is, so long as I do not distribute any of this in any way (the game, the source code, or any part thereof), is there potential for legal repercussions? My main activity within this is to make small changes, recompile the whole thing, and to host a server using the new software to provide a unique experience (when compared to normal server hosting software). My second question is this: At the same time, I would also like to be working on a game of my own design. It shares a small number of very minor things to X: it is 2D, it has minor RPG elements, but not in the same type or scale. And that's it; it is not written in the same language (computer programming language; what form the source code is made); it is going to be an MMO, while X is smaller scale adventure; it is isometric, while X is side scrolled. Would there be any reason which this is exceptionally prohibited?