In theory, because unreal now saves every instance of actors and objects in a unique file, it should be possible.
I have not tried this myself and i have no idea what kind of issues it could create. If you try this definitly make a backup of the project first and expect to encounter some small issues after even if it does work.
Locate your project files, which should be in something like
“Fortnite Projects/YourProjectName/Plugins/YourProjectName/Content”
Theres two folders called ExternalActors and ExternalObjects which im pretty sure corresponds to the asset instances in your level. I would recommend deleting half of them and then trying to launch your project. If it doesnt work delete the other half. If it does work, keep halving the assets you delete, until you narrow down the issue.
Most likely the issue is in the ExternalActors folder imo.
Again i dont know if this works or not, use it as a last resort and only if you backup your project first.