It’s not clear to me how a project gets flagged as a TMNT/LEGO/etc island on Epic servers. So unless you can find a way to remove the tag when viewing it on the create.fortnite.com interface I don’t know if changing a template type would be possible.
Try creating a new non-tmnt template. Copy the content from your TMNT project C:\whereyoustoreuefnprojects\yourprojectname\Plugins\yourprojectname\Content into your new project. You can also try having both projects open at the same time and copy-pasting between them.