Hi! Not a full answer but MS DOS is OpenSource with MIT license that means -you can do whatever you want basically. Embed and sell too. It’s available trough GitHub. It is 1.25 and 2.0 tho. You can use basic C++ features to call any external app. Or just embed it all inside your project, I don’t know how to make GUI inside UE4 - I will look into it a bit more if I find something I will comment here.