You could make a digital copy of the tape by connecting the cassette player to a computer (or if that's not possible, use a microphone) and using software such as Audacity (which is free). Once you have a copy on your computer you can stop and start it as much as you like without risking damage to the tape. You can also slow it down, although while that can be useful for working out how a melody goes it might not make lyrics any more intelligible.