With some work, yes. You have to copy and overwrite some map files to get rid of the infrastructure, but it is possible.

Maybe someone knows how I can do this? Since I am not competent in such matters.

Here is how to do it:

1. Open the terrain editor, make new a map, save it, and exit the game.
   
2. Go to wherever you installed the game and open the "media_soviet" folder.
   
3. Go to the "campaign2" folder and copy the following files:
   • collision.dta
     
   • emissivemap.dds
     
   • farmap.dds
     
   • grassmap.dds
     
   • headerE.bin
     
   • heightmap.dds
     
   • mask.dds
     
   • maskdetail.dds
     
   • material.mtl
     
   • material_fall.mtl
     
   • material_winter.mtl
     
   • quadtree.dta
     
   • resourcemap.dds
     
   • resourcemap2.dds
     
   • trees.bin
     
   • treesgame.bin
4. Go back to the "media_soviet" folder and open the "saveterrain_editor" folder.
   
5. Open the folder with the name of the save you made in the terrain editor.
   
6. Paste and overwrite the files you copied earlier into this folder.
   
7. Restart the game and open the map in the terrain editor. The name of the map may have changed, but the folder's name should not have.
   
8. Replace the customs houses and foreign power connections that got erased. You can find these in the "CIV" tab on the right side of the bottom bar.
   
9. (Optional) Use the "Restore soil" tool to erase the disturbed dirt that was under all the roads and buildings, then press the "n" key a few times to grow the grass over it. You can find this tool in the "Terrain tools" section of the bottom bar.
   
10. Save the map as a new file to change the name to whatever you want.
    
11. Delete the folder of the old map that you copied the files into.
    
12. Start your new custom game on the new map file (should be one of the last in the map list with an icon consisting of a red wireframe landscape and the words "Terrain editor.").

Thanks, it really worked

You're welcome.