Repairing those damn switches in cassette decks??

[P4DQt at aol.com]

How can I fix "Tescam Porta 2" 4 track casset recorder?, it has the automatic 
push a button and it pushes the head into the tape by means of gears and 
cintrifugal force, so that is like automatic and not finger force, that makes 
the tape machines playback head touch the tape when one presses the play 
button (Know what i mean?)....  Well one of the plastic gears in this 
mechanisim is worn off from use, so that the oval shape on the side of one of 
the plastic gears has worn down from friction and does not do what is 
supposed to do, there for not letting the automatic use of the buttons one 
presses to get the cassette to play, record, rewind, pause, etc, I need to 
replace this plastic gear, how can i purchase one? 
  An up front thanks for all the support the lists provides
Sam