What editing soft are are you using? On Sound-Forge (etc.) you can generally normalise the L & R channels independent of one another, though if the channel is seriously weak the S-N ratio might be obvious.

I had this problem on an old deck; a mate did a tweak with a screwdriver inside which fixed it for a while but it died shortly afterwards. Think seriously about a new player, if such things are still available in this digital age...

Good to see a couple of cassette threads anyway; as I might have said elsewhere, I alays back stuff up on cassette just so I'll have something to listen to in 20 years!