Don't do it !  
Old cassette decks contain perishable components -- slip-clutches, rubber belts loads of plastic components exposed to heat -- they just don't last that long -- I have had Sony, Teac and Nakamichi models which will testify to that. 
Some parts for some brands can still be found but disassembly is not something I'd tackle -- and I've stripped down several Revox reel recorders.