You might find that a standard cassette player motor can't come up to speed (or change speed) quickly enough (due to a flywheel most players use); plus internally (on some players) they have an electro-mechanical "governor" (of a sort), internal to the motor, which uses centrifugal force to open contacts or force the brushes away from the motor to keep it at a fixed speed. You would either need to bypass all of this, or use a different motor (and ditch the flywheel, etc). If you can byppianopianoass the flywheel (not easy because it's shaft is what the tape passes between - that and the pinch roller), and use a different DC motor (or bypass the governor - not too difficult if you have any experience taking apart and fixing/modding small motors) - that would probably be your best solution. However, if you've never disassembled a cassette player before, you'll be in for an interesting mess on how to do it. As a kid, I used to tear apart many broken cassette players relatives and friends of the family would give me, so I became quite familiar with their internals. The mechanisms are all pretty much alike, but they still tend to be quite complex mechanically, and not too open to modding much.