[jezek2]

It doesn't matter how hard it was to produce, it is still a derived work. Binary vs source code doesn't matter for copyright, it's just a different form of the same work.

It matters with patents where you can have patented stuff in the source code without a problem as long as it's not compiled into the binary. Users who are unaffected by the patents or have a license can compile it in.

Fair use is used during a trial to determine if your use case is an OK exception despite being it a copyright infringment. It's not guaranteed to go into your favor (there are a lot of nuances case by case despite of precedents) and you need to be sued to exercise it.

What you can do is to mimic the behavior and look from scratch, then it would be a separate work. The internal structure would be quite different too. Even the same logic would most likely be coded differently. And the look wouldn't be pixel-perfect just very similar looking.