I've used SDL1.2 (DirectX 7) to port the DOS version to Windows. The black borders on full screen mode are due to video hardware not being able to scale up from 320x200 - some monitors / video hardware can do it full screen no problem, some others not - it's not something you can change with software. There's no hotkey or quick fix because the game's graphics are hard coded to a special (unchained) 320x200 DOS video mode.
I can re-engineer the graphics to scale using SDL2.0. I was thinking of crowdfunding to pay for the time it will take to do, is that something you are interested in donating to?