Does the axe seem to have plenty of power but just slow? If so you could reduce the size of the sprockets on the axe or increase the size of the sprockets on the motors.
Or if it seems to lack power you could try the reverse (increase sprocket size in axe) which could speed it up if the motors where under too much strain as they want to run at a higher rpm.
Other idea is shove a powerful brushless motor in there, but not sure how well that will go either.