Sadly, the most likely problem is the playback head needs replacing, and the cheapest solution is probably another machine.

You may get lucky and find it's just a dry solder joint somewhere; if you're technical enough to do that, maybe replacing the head could be an option, as they aren't expensive - unlike my Teac 3440 open reel four track (currently a three track!), which will need around 600 GBP to bring back to spec - unless I can find a secondhand donor machine with a different fault.

Lining up the azimuth (angle and position of the head) is nothing like as hard as some folks would have you believe, though you will need a tape with a test tone recorded on a properly aligned machine to be certain of good results. It can be done with a commercially recorded music cassette if you're careful.

The ones I found a bit tricky were those auto-reverse units with a flip head, because the transit path usually needed tweaking as well.