I always use
Sound Forge for correcting single channel to dual channel mono. I can open the AVI direct in
Sound Forge, copy the good channel over the bad, save as WAV. I can then open
VirtualDub, stream copy the video, with this different audio file, to a new lossless AVI. In some cases, I don't even need to correct the lossless, the WAV is just used as source for the latter encode.
This is actually a very common problem. Camcorder "stereo" was farce that had many issues not noticed until the digital conversion era.