OK, you'removed the smarts from the servo.
For one direction control & speed, you need to use PWM with a MOSFET or motor-driver shield.
For bi-directional control & speed, you need use PWM with an H-bridge or H-bridge shield driven with PWM
Remember (in general) any motors cannot be driven directly from the Arduino I/O pins.
They should have a separate +V, otherwise, you could overdrive the on-board voltage regulator on the Arduino if the motor is stalled or pushing too much load.
(The internal servo PCB previously addressed that.)
Must have common GND connections, or you can toast the Arduino CPU chip internals