I wish I knew more about electric motors.Trying to "hop up" that motor is the way to go.The little gas motor probably doesn't have a lot of potential, but N2O would be the way to go if you wanted more power cheaply.I'm not sure how it would affect the integration of the two motors.
Slot car racers did a lot of hopping up electric motors. You could get a second battery pack to "run" your hopped up" motor.
The fly in this ointment is that the Honda electric motor is probably a pretty high powered already (relative to weight and size).
I would consider the N2O, but I just don't know how it will integrate with the ECU etc.
You might try simpler hotrod stuff like lighter wheels, and small diameter wheel/tire combos. I have seen relative cheap($600 set) wheels in the 10 lbs a piece range. Luck,Charlie