Copy and rotate the entire map is simple enough : Ctrl+alt+leftclick to make a selection window-always on top view- the objects selected are highlighted (don't select the skybox), unselect the exterior brush by leftclick while continue to maintain holding ctrl> release all keys and rightclicking on the selection (zoom if needed with wheel mouse).
On the menu choose Duplicate (don't make a left click during the whole operation), now rotate ctrl+rightclick+moving mouse (enlarge the top view window) the selection will rotate>align on the grid>180 degrees(to the eye), always hold crtl then leftclick for moving at the good place

. Unselect by leftclicking on an empty place > it's done!
You've got now two terrains

the new is TerrainInfo0.
The problem now is that the terrain never rotate because it use an heightmap and layers which are in the texture browser (mylevel), you must use
Utrota, all explanations are in the text file.
For this you must place it in a folder and export all the layers and the heightmap to this folder, then make them rotate to 180 degrees by the dos command :
Examples:
cd : your hdd
cd: your folder etc..
utrota redbase1.bmp redbase1r.bmp 1
utrota RBaseSnow5.tga RBaseSnow5r.tga 1
You must afterwards reimport them in your textures-mylevel and modify the TerrainInfo0 properties -in our case- (binoculars search for actor)>array TerrainInfo>Terrainmap and TerrainInfo>Layer by highlighted the corresponding textures on the tex-brower and click use in the TerrainInfo properties at the right place

> Rebuilt: it's done .
You could see now that there is still some invisble piece at the location of the trench (and floating building), you can fix this with the invisibility tool of terrain editor (make sure you choose the good terrain cause now there's two:).
You may use painting (not 100% but 15% of strength and 200 radius) to move the terrain up and down point-by-point, and finishind the work with smoothing (1%) to join with the bottom of the buildings, you may also move down the floating ones.
Don't be afraid to save your map even after a little work with increasing number example: v1,v2,...v158 ,last thing if you make a big hole in the terrain >don't panic

just use Flatten for making everything back to a flat.
Hope these humble advice may help you.
Oh, the exterior brush like all brushs must be scale by rightclick>transform>scale.