Did the lav track go away completely or is there something understandable there? If not, then I'd try to line up the Zoom to the good lav audio just before the problem area. That will get the Zoom audio to the right area of t he track. The next audio should be what's missing. Then, look for a mouth movement early in the take that corresponds to a sound on the track like an "m" where the lips are closed. Slide the track frame-by-frame until the sync looks good. What a pain. Good luck.
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