Can I copy a game?

Hi Guys, My name is Serghei, I live in Moldova and I have a question: Can I copy a game's functionality legally? What I mean is that I want to make a game based on another game's idea/functionality. The game I am referring to is http://utopia-game.com initially my game will be 90% the same, in the future I will make changes and it will become slightly different, but initially I want to keep it simple. I don't want to copy the game's graphics or soung, just everything else Thanks for your time, have a nice day