Just recently I tried running Deus Ex and had to use the hacked .exe files and a custom graphics mode for it to not have serious issues.
Tim Sweeney said he'd like to release the Unreal 1 source code one day (5 years ago) and it's stuff like this that would be sooooo much better even if we could only get Unreal itself and not further forks like Deus Ex.
(For instance, with doom modding they had to totally reverse engineer STRIFE but it properly works on modern systems now!)