I’m pretty sure you can always modify code for personal use, you just can’t always distribute those changes. In the case of a game engine, this would mean you could modify the engine code in development, but you could not release your game with those changes in.
Unreal allows modification and distribution, but only if you’re a licensed user and only for your combined work, but you cannot distribute your own fork of Unreal, aside from a patch set for other developers.