I'm assuming this is akin to the http://www.openttd.org/en/ http://www.openttd.org/en/ or Doom clones where the "engine" is open source and you have to provide

I'm assuming this is akin to the http://www.openttd.org/en/ http://www.openttd.org/en/ or Doom clones where the "engine" is open source and you have to provide your own graphics/sounds/etc. As a stop gap until they completely replace the art assets with OSS version, you can extract the originals and use them for development. That requires an original installation to coexist with the clone (read https://github.com/fabianschuiki/OpenSkyscraper#how-to-try-stuff https://github.com/fabianschuiki/OpenSkyscraper#how-to-try-s... which mentions you need the SIMTOWER.EXE locally, mainly to acquire art assets). So, you need a legit copy of SimTower to play the SimTower clone (at least until they rebuild all art assets).