I would suggest a way to shift the motor. So you have a low gear for high torque to get moving, once you get moving shift to a high gear to increase your speed.
Also, increase the voltage to the motor. is it 6 or 12 volt? 6 volt motor could easily be taken to 8 volts, and a 12 volt motor can run at about 15 maybe 16 volts. The motor will run faster and possibly a bit hotter so we will need to see of the trade off is worth it. I have some old motor run caps too that can help give it a boost on start.
Some things are meant to be closed. Your mind isn't one of them.