Unity it is not just usual 3D content, here you can also use canvas and sprites too. It is possible to make FNAF in Unity and some people already did it. On this page you will find full version of Five Nights at Freddy's: Part 1 made with Unity. It's full-featured recreation of known horror game. Enjoy!
If you interested how to decompile FNAF frame by frame just take a look here - you will find some hints how to do that.