If you cant do it yourself, just go back to the people making the engine and ask for a fixed version?

If they dont, try to find a way to overcome the unmentioned problem why you cant recompile.

Otherwise discard it or start hex-editing, ... ?

Just reporting back that the problem was a missing keyword in the window style. It didnt matter for earlier versions of windows. It now works in newer windows as well. (i got hold of one of the creators of the engine).

Thanks for your input!