I haven't looked into licenses yet that much, but if you are afraid that someone is going to 'steal' your code to make money out of it, then my advice would be not to release it.
No matter what license you put on it, with different graphics, other sounds, new levels and a few functional changes in the game itself, not that many people are going to see the similarities beteween your game and 'theirs'. (Especially because so few know your game)