I'm not certain why I was downvoted. Sure Unreal Engine 4 isn't free and 100% open source(unless you pay a measly $20), but pretty damn close.
Two requirements were that it must be open source, and it must be made for 2D.
It is not open source. It is not 2D, although with some work you can constrain the viewpoint if you wish.
Unreal Engine 4 is 2d capable. Paper2D was released with the latest version (4.4)
So apologies about open source. I assumed he simply wanted access to source.