Here is also a website where you can see if there is an open source port to non-open source games or an alternative that e.g. tries something of its own / based on the gameplay. As an example, the following “clones” are listed for Minecraft: Minetest, Mineclone2 (now VoxeLibre), ManicDigger and more.
Edit: fixed some grammatic fails.
Unfortunately, it is broken and there is an issue about it on GitHub.