On a practical point of view for cassette repairs, for quite a few years I've always refused to get involved in repairing them - it's rare that all spares were available, and was cost prohibitive even if they were. Many 'later' decks were also very difficult to work on, and it only made sense to replace all belts, pulleys, clutches, pinch rollers and motors while you were doing so. Invariably, the heads also needed replacing as well, as most heads are soft and soon get a groove worn in them - running your finger nail up the head soon shows if there's a worn groove on the surface - this results in loss of treble, one channel much weaker than the other, and poor tape tracking.
Basically a tape is a very fine file - pieces of metal stuck to plastic - and this soon wears through the soft steel of most heads. The actual 'head' itself is made of ferrite, which is much harder then the steel, but is only one half of the tape width (bottom if I recall correctly?), hence the uneven wear across the head.
Years ago I had a Sony TC-186SD cassette deck, and these used 'ferrite and ferrite' heads (which were particularly expensive), but because all parts were ferrite they didn't wear. Mine had a huge amount of use, and the heads were still like brand new. In later years I replaced it with a Kenwood deck, with three heads - but that never got a great deal of use, although it sounds great, as you could adjust the bias while monitoring from the third head to get the best sound.