This is exactly what we love from you and your creations; updates to AC that might seem trivial but really brings the immersion, reality, and general quality of the game to another level. You managed to fix what I felt has been missing from this great track. Always looking forward to see your future releases!
Open the track as an .fbx file in your preferred 3D modelling program (I prefer Blender), add a simple plane (or just a single square shaped face can do), select a material for it that is compatible with transparent alpha (a track material surely has at least one), duplicate it so it will be standalone, then modify the UV as you wish and make a .dds file for the texture you want it to show like the city or a mountain or something. Then when you finished delete every mesh that is from the track mod itself, only the plane mesh you modified should remain. Then export as .fbx don't forget the FBX scale, import it to ksEditor, edit the properties like Diffuse and emissive as you like, then save as .kn5.
Open the models_*TRACKNAME*.ini in the track you want to modify, and just add a new line with the next number in it like:
[MODEL_9]
FILE=*NAMEOFKN5*.kn5
POSITION=0,0,0
ROTATION=0,0,0
If you did everything well it will be visible in-game :)