IIRC, the usual workaround for asset licensing is to make an automated extractor for people who have the original game. This buys you time while you work on drop-in replacements.
https://archive.org/details/SimCity2000_Special_Edition_Maxi...
Specifically: http://web.archive.org/web/20070806010730/http://www.fmod.or...
It's very likely to not work on modern systems anyway (dosbox will work of course) and version from GOG likely wouldn't be installable on let's say DOS or Windows 95. And there is plenty of people who might want to run old games on some ancient PC for personal fun or for museum-like purposes.
Another option for open source projects it's to just use demo version which is 100% legit way to get assets. Though it's not always possible because some demo versions don't contain whole assets that fully-featured engine would depend on.