You can still buy head demagnetizers but they're more rare these days. They're more needed if you record much. Playback doesn't tend to magnetize the heads (although it's still a good idea to demag the heads at least once initially). Demag is important as a magnetized head will gradually erase any tape you play on that deck (especially the highest frequencies).
And do NOT use acetone for head cleaning. A lot of heads are laminated with glues that are softened by acetone and you may ruin (the likely irreplaceable) head. Likewise don't use acetone (or even alcohol if you can help it) on the rubber pinch roller as both destroy and/or harden rubber.
Cassette decks are tricky these days. The old good ones are mostly impossible to get parts for and their rubber parts (pinch roller, belts, idlers, etc.) usually dried out a long time ago and heads might be badly worn. And almost all the new ones you can buy today are mostly made-in-China junk by comparison to the better old ones. They tend to have audible wow and flutter, lousy frequency response, poor azimuth tracking, lots of hiss, etc. They just generally sound bad. And the noise reduction in modern cassette decks typically doesn't track well with NR in vintage machines. So if you're playing back vintage tapes, that further screws up the sound quality.
