Here's something interesting, an open source Warcraft II style engine https://en.wikipedia.org/wiki/Stratagus
You'll have to adapt the same strategy as they do, or any other engine remake: Host and share your engine, require the user to themselves supply the original game installation. That way your project is just a mod.
Another example of a great engine remake is "RttR" http://www.siedler25.org/ a remake of the Settlers II engine. They require a separate installation of the original game to play.
(Still some copyright issues remain)
Little correction needed. "Mod" usually mean derivative work of original game, but open source engine projects are not derivative work.
So it's just an engine that have nothing to do with original game or it's copyright holder.
[1] https://en.wikipedia.org/wiki/Command_%26_Conquer:_Red_Alert...
It is cool and impressive what you've achieved here, don't let that discourage you. Keep building these things.