You can fake the old asset.
In Content Manager find another switch machine and open the folder for edit.
Go to your Trainz data editing folder wherever that is. Mine is in E:\TRS22_DATA\editing, for example. You will see the asset you opened. The folder begins with kuid
Copy that folder to your desktop.
Revert the one you opened to original in Content Manager.
Don't close Content Manager.
Open the folder on your desktop and
Open the config.txt file for this asset and change the KUID to the one that's missing.
You might want to change the username to something else, so you don't get it mixed up with the original - add in your initials or call it my replacement or something.
Save the config.txt and close the folder located on your desktop.
With Content Manager still open, drag that folder on your desktop into content manager.
This asset should submit fine, and your once missing switch machine will no longer be missing. At this point, you can now do a Bulk replace assets on that switch machine to a new one of your choice.