Hey, you can find the “TheIsland” map in the \Maps\TheIslandSubMaps\ folder.
I think at the moment you would have to recook(and upload) the whole map if you want to make changes to it.
Look out for the next devkit update as they announced a feature that should make this easier:
- Capability for mods to overwrite or add-to the ARK Island maps, which enables much more dynamic map mods (stackable too!)