the line for speed is 26 the analogue read that value is divided by 4 try higher numbers like 6,8 but the actual speed is set on line 46 the analog write you can try to remove that variable 'motorSpeed' and replace it with values from 0 being off to values up to 255 . e.g. analogWrite(enablePin,100) but doing so will disable the pot. Other alternatives use a lower voltage battery other than 9v. things to research is pulse width modulation and also the L293 IC as to how it works and pins used. There are a lot of tutorials on those subjects.To use gears etc would be a pain unless you get a geared motor. one way to debug projects with variables is to use the serial monitor then print the value of variables like motor speed to display on the built in serial monitor of the Arduino IDE see project 3 in projects book its shown there. use a Serial.print(motorSpeed) because values around 10 should slow down the motor