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.