It's more complicated than that. For an asset to be derived work from an original, it is not necessary for it to contain anything from the original. If you start from copyrighted assets, and meticulously replace them all with your own art piece by piece, while following the style and constraints of the originals, and while looking at the originals, I'd bet that a court would find your work to be derived from the originals and therefore under their copyright.

A lot of the fan-driven reimplementations of classic games are trivially derived works, because people seem to think that the copyright only covers the pixels in the originals and if you replace them you're fine.

FreeDoom does that with Doom and it has compatible assets but not in the same style altough they are done in such smart way that most PWADs and TC are totally playable without clashes, from Requiem to Back To Saturn.

On game engines, reimplementations are not derivations at all but tools for interoperability, totally legal to create. From Wine to most of the stuff of https://osgameclones.com, to GNUStep against NeXT/OpenStep API (and Cocoa from early OSX) and so on.

If you could sell Cedega back in the day you can totally sell OpenTTD with free assets, period.

The entire PC industry exists today because of cheap IBM BIOS clones from Taiwan.