Crossfade.
I don't have time to lay it out in great detail, but I'd take advantage of Audacity's multitrack capability. I'd save the good L channel section as a new file, import it into the project and set it's properties to Right Channel , align it with the original bad R channel section, crossfade at the beginning and end of the area I patched, mute the bad R channel section, and mix down to a stereo file. Basically, doing it this way with the crossfade makes for a smoother transition from the stereo portion of the file to the mono portion of the file, and vice versa.
The reason you need the 3rd file is because you need to overlap the files in order to crossfade, like this:
Line up the two WAV files in a multitrack/montage editor, so they overlap like so:
[----------track 1, primary source, missing first min-----]
[-----track 2 w/ first min intact----------]
Make sure the two tracks are time-aligned for the part that overlaps by picking an easily identifiable section of each waveform to line up. Then, apply to the parts that overlap: crossfade-in track 1, crossfade-out track 2.