On the good portal I needed to set the value
Pcd.TransportApplication.Export.ExcludeSystemObjects = false and on the damaged portal I needed to set Pcd.TransportApplication.ProtectedNamespaces = com.sap.km (removing the com.sap.portal option)
I could now create a transport on the good portal and add the page Application Designer Framework Page from pcd:portal_content/com.sap.pct/admin.templates/pages, export it and import it to my portal. After the import, the editors now look OK, although not as they looked under SP11. However, I’m prepared to lose some screen real estate in the interests of having a working portal!