..on Amiga?? Do you know where I could download it? I had every MK2-version but I didn't know of any Amiga port! Is it pretty much the same as the PC version?

On the PC the Deadpool fatality only worked with my pad, if you held down LP+LK+Uppercut on the keyboard the move wouldn't be recognized. (Or you had to disable Numlock if you played with this layout _err or something...)

I once spent hours trying do get this fatality to work, but I had no success, I think it's not possible to perform it on Amiga

Have you tried doing character specific stage fatality inputs instead?

IT'S POSSIBLE!! You must perform uppercut down + fire) and at the same time turn joy to opposite down/left or down/right direction. I mean opposite to your opponent .You must do it very quickly!