It seems to me, if you use a different style with your own textures and models, that should not be a problem with copyright, because you did it yourself, right?
That would be correct....
Anything you make yourself you can use. unless its Trade Marked. (TM).
People can only trademark / copyright a design.
If you redesign it and make it your own way and can prove youve made it all (yes this includes music and code not just textures / models) then theres not alot anyone can do about it.
take facebook as an example. they tried to sue the person that inveted it but didnt get very far because they couldnt prove he stole any code...thats because he created it all from scratch.
Why would you want to use somones levels as inspiration though, surly you would want a unqiue level to go with your unqiue game?