If you own Half-Life 2, you get the source SDK, where you can mod Half-Life 2 completely to another game, and you can modify anything (engine, HUD, etc.) But, that requires some (
some) C++ expertise (programming) , compiling, scripting and probably you want to have your own sounds, which also requires a studio with good recording software, a good microphone... AND, making a game takes A LOT of time, they worked with about 80 people on Half-Life 2 for five years, so you'll surely know what to do...