Hard.
0) If a game is running on a multiplatform engine, chances are the studio is already selling a PC version, rendering this whole thing moot. Otherwise they're likely on an in-house or licensed single-platform engine.
1) "Popular game engines" are highly customized by the studios using them (with the large studios, often substantially). Though they may share commonalities, you'd nevertheless need to tailor your modifications to each and every game.
2) Even in a game engine that has bundled asset packages, scripting languages, etc., much of the core functionality that makes a game different from another game, including some of the gameplay code, will be in the C++ or other compiled code.
3) You don't have source code to the engines in question. Major console games aren't made on open source engines.
4) If you modify and redistribute engine code without permission, you're very much on the wrong side of the law. Your development will have to be in secrecy and you'll have to distribute anonymously, while very aware that someone will subpoena anyone you distribute through to find the source. Good luck finding recruits.
5) If instead you try to create your own black box recreation of the engine, well, uh, again, good luck. Modern game engines are beasts.