Speed up or slow down motor of cassette recorder using a potentiometer
In many of these old-school devices the motor speed was controlled by a centrifugal governor on the rotor of the motor, so faster was not an option only slower.

I built this: it worked well enough to be a fun toy:







simulate this circuit &ndash; Schematic created using CircuitLab

it just plugged into the 'Cue'  socket (J4 REM on your schematic) placing it in series with the motor. It did not give stable or repeatable speed control 

Any reasonably big power transistor should work eg:TIP31 the 2N3055 was just what I had on hand.