Yes, you will need to restore ReportServer and ReportServerTempDB. All of the report definitions are stored in the ReportServer in encrypted fashion. However, after you restore these DB's and start SSRS I believe a new record will be inserted into the ReportServer.dbo.Keys table for the default instance. If you open up that table, you will probably find 2 records - one for the current default instance and one for the old named instance.

There are 2 things that need to happen next:

1 - The old instance record must be removed from the Keys table so SSRS doesn't think we are creating a scale-out SSRS farm

2 - The new SSRS encryption key created for the default instance cannot be used to decrypt the report definitions encrypted with the old key. Hopefully you have made a backup of the SSRS key from the old instance. You can restore the original encryption key using the Reporting Services Configuration utility.