In fact it would maybe be a good idea to restart from scratch cause the code is pretty ugly and the Gui is not very flexible.
I was afraid you were going to say that, lol. Creating level editor can be very demanding and I personally do not desire to take on the challenge. Developing the GUI alone can be a major pain.
On the other hand, the community could benefit from an FREE open source Map Editor. If you elect to start from scratch, perhaps the GUI can be omitted. If you elect to include a GUI, I would recommend using a 3D GUI, that could also be used for designing in-game huds and control panels.
Another alternative is to contact developers who have made editors and try to get our hands on the code. The biggest challenge with this isnt getting the code but, understanding it.