You might try my TOCSaver plugin but I have not played with it in years:
https://www.mobileread.com/forums/sh...d.php?t=333097
Please read this first:
Before Running on ePub3
This plugin will only parse the NCX ToC. So if you have an epub3 and want to save the Nav ToC, then use Sigil's ePub3 Tools to generate an NCX from the current Nav before launching the plugin.
Notes
Since this plugin could make changes to many xhtml files, it would be either a good idea to make a CheckPoint before running the plugin, or at the very least do a File Save-As under a new name, to make sure nothing is ever lost.