You can keep the original menus. Reduced compatibility can happen if you try to retain the original menu/VOB structure. It is best if you reauthor to retain assured DVD compliance.
The problem you will most likely run into is the speed of the audio for both sources. Most analog devices will record at slightly different speeds, which will cause the new audio to drift out of sync. This is why you will need to edit the FM audio in a multitrack wav editor to make it match the original audio. I can't tell you which is the best way to make the FM audio match, because for me, the amount of "drift" will require a different technique for different situations.