Most medical scooters are designed for a maximum speed of 4-6 mph. This maximum speed is for rider safety and stability. 4 mph is faster than a person can walk and most people operate their scooter at 1/2 to 3/4 maximum speed. They cannot be modified in any way to go faster than the maximum speed set by the manufacturer.
Scooters CAN be adjusted to reduce its maximum speed. You would consider this if the rider is unsure of themselves or concerned that the scooter may go too fast.