Hello again!
Glad you managed to sort part of the problem with a new project. As for migrating elements from City Sample, that can be quite a daunting task on its own.
The map has a lot of plugin dependencies, and it’s loaded with heavy assets. Still, it should be possible to migrate just the level you need.
Start by loading your city level, and isolating all related assets to it. From there, use the Content Browser to locate their source files, and their respective folders. Now, use the Migrate function to send them to your active project. You can find further details in the doc below:
Additionally, a similar scenario shared the following tidbit related to migrating elements from City: