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.