Yeah, I would get the source code.
First of all, you'd have the full game source. In that way, you'd get the map editor source code as well. (The map editor was actually compiled into early builds of the game, I think. Probably just controlled using a build time switch).
The only thing you MIGHT not have readily available would be the graphics files (well files, op2_art files) but you have a copy of the compiled, final game with those so that's not an issue.
Having the source code, you could do most of the other things in the list (make better AI, get the map editor, etc)
It would also make my 'hacking' obsolete, because I could just read the source. (Caveat: I didn't know C / C++ back then
)
One thing that would also be nice, is to build support for WON, or even advertise OPU on WON. (But again, anachronistic: OPU didn't exist back then).
Interesting things to think about. If only...