Could you run a power wheels with no fuses. if so, could you run a lot more power through the electric motors. could you make it faster with out blowing the motors which are rated for 12V.
You can overvoltage the motors, but you must pay attention to current. Once you start drawing 2x or 3x the current continuously, you really stress the brushes and windings. Odds are the motor will not last long under these conditions.
Also, the Power Wheels motors are VERY small. They depend on a huge gear reduction to the tire. This makes the vehicle move very slowly. I wouldn't waste time with these if you are building a go-kart.
Find a circular saw motor, a treadmill motor, or a wheelchair motor.