Hey Neil, loving your game and my only complaint is that its a bit small so I was wondering if you could send me the code so i can turn it into a bit of a coding project over the summer? I have absolutely zero idea how to code so this is probably going to be quite difficult but also probably quite fun!
I sent you an email. I think getting the actual game code to compile is going to by a struggle, as it's old code in D and I expect the D ecosystem has changed since then.
But the missions, and I think the aircraft, are set up in plain text files (plus some 3d files for the aircraft models) and should be fairly self-explanatory. You can test scripting commands by opening the console and just typing them. There's some discussion of it further down this comments page, from six months ago.