Personally I wouldn't use any paid artwork in an open source game unless the license is compatible with open source. The problem is your game cannot be distributed with any Linux distribution when the assets are restricted. I recommend looking at opengameart.org and see if you can find something equivalent there. It can be hard to find things that match up in style but it makes it easier to distribute your game. Open source code that people can't easily expand on isn't of much value in my opinion.