Making a legal argument that a game clone is not a derived work when the source code was literally derived from the original via development over time is a pretty sketchy argument. It's not impossible -- that's sort of what the first open source BSD versions were (AT&T code was replaced piecewise) -- but I wouldn't bet on it.

Cloning of interface and operation (but not artwork) is widely held to be legal, but in general all successful clones are clean room clones.