If you are in a sandbox or parallel when using ue4 you’ll need to have it open when trying to work with explorers file system.
As long as both your corrupt project and the new blank project are the same version (4.8.1 in your case) you should be able to go to Documents>Unreal Projects>Old Project(the corrupt one)>Content and copy all but the maps folder and paste it into the new project. Located at Documents>Unreal Projects>New Project>Content
I’m not sure if it matters but I would close the Epic Launcher while copy and pasting the files, just to be safe.
When you launch the project it may take longer then usual to load in the new assets, it should work though. I just tried it to double check and it worked for me.