[LeoPanthera]

As an alternative, perhaps an artist could create new free assets for use with the engine, which is what happened with OpenTTD.

[unixhero]

And as happened in OpenRedAlert and OpenDune.

Remaking sprites and game assets in a more open format - while also supporting the original game assets.

[rplnt]

OpenTTD did this, eventually. But for years it used the original assets. But it was your responsibility to get the game.