Some cassette motors have a built-in electronic speed regulator. Some are external power transistor/board and extra wires for the tach.
Look very very closely at the back of the motor, in the holes and see what they are about. Sometimes there is an adjustment trimpot.
With the older centrifugal weights speed regulators contacts can get stuck. I'm suspicious there are no capacitors outside this motor.
You can measure the motor speed or listen if it changes with a change of say 12-14V power, that will tell you if it is just regulating fast, or no regulation at all. I have taken motors apart and done surgery when desperate.
I am finding those grey electrolytic capacitors are drying up as well, so make sure the motor has clean power.