Dealing with phase

I've been digitizing some old cassettes, and on one of them I noticed that the left channel was much quieter than the right. My first thought was to just sum it to mono, since the original recording was mono, but that just makes most of it silent. I exploded the stereo to two mono tracks and flipped the phase on the right channel, but this just makes it worse. How do I fix this?

If original is mono just choose the one track that sounds the best and mute the other - or? Then use whatever enhancing plugins and stuff on that. Noisereduction, add air by adding highs etc. You can even make it stereo from mono with plugins like Waves PS22. :)

Yeah, I guess that would be the easiest solution, thanks. I had I feeling I might be over-thinking it.