OpenRA is reimplementation of the Red Alert game engine that support multiples games. They have their way to deal with the assets issue but I forgot what exact solution it is. At least RA is available for download for free for EA so its help too.

Edit: « OpenRA is 100% free, and comes bundled with three distinct mods. When you run a mod for the first time the game can automatically download the original game assets, or you can use the original game disks. »