Many ways lead to Rome. 
Ther easiest way would be to load both files or collection of files, the flac ones and the mirrored ones, into MP3tag at the same time.
Sort them in such a way that the ones wit rating are grouped together and the ones without. Also make sure that both groups have the same inner sequence.
Then select all the ones with rating and copy the tags. Then select the ones without and paste the tags.
This is the easy, local way.
Also, there is the possibiliy to export filename and field to a text file.
You then edit the exported text file and replace the file extension from flac to the other.
Then load the files without rating and select them
Now use the Converter Text file - Tag.
This should put the exported data into the named files.