In digitizing a cassette tape, I found the sound was bad or partly missing on parts of one track. I reduced the volume on that section and turned the good track mono so it would play on both speakers and mask the problem track. This makes the whole good track mono all the way through, and I would prefer to make it mono just on the selected part where the other track was bad. Can I do this, and how?

Spit the stereo track into two single channel tracks (Click on the track name then select “Split Stereo Track” from the drop down menu).

Fade out the bad bit on the appropriate track.
Duplicate that section from the good track and apply fade-in / fade-out to the ends of the duplicate section.
Click on the name of the duplicate track and change the channel (left/right) so that it is the same as the bad track. You have just made a “patch repair” to the bad track.