Which way will the track be oriented with respect to the Sun's position in the sky? You'd have to align the solar panel to be as close to NORMAL to the Sun's direction in order to get the best electrical power output from the panel, consistent with wind drag. The elevation will be fairly constant for the day, but you might want to make it possible to point the panel in the horizontal plane.
(Perhaps a fixed horizontal position would be the easiest, if you want to complete it by Christmas!)
Then I'd imagine that using Maximum Power Power Point tracking will maximise the available power transfer to the motor. This would involve a pulsed DC drive to the motor, allowing you to make the internal resistance of the motor+drive to equal the internal resistance of the solar panel.
After that, it looks like you are in the lap of the gods where the motor efficiency is concerned, as it seems you can't choose the motor. Fair enough, but who says which motor out a box of 100 is the best?
Finally, make sure the tyres are good and hard!