legality and copyright is only an issue if you attempt to make money off the mod. you can pretty much make anything you want as long as you don't sell it to multiple people. you can even sell it once, for example, you could make a sports team t-shirt for someone and as long as you make one and sell only that one, it's legal. same applies to this sort of thing. so don't worry no one'll get in trouble. long as you don't ask for money.