Overriding/changing properties of things aleady within the game.

I want to make a furnace that renders things inside it like the fuel source and item of interest, but I don't want to make it a new block. I wish just to change the current furnace to have these features but I am puzzled as to how. I would also like to know if there's a way to look at the Minecraft source code within Mc Creator because I presume that would be helpful for changing things currently already in the game.