motor boost

the casing on my motor is stamped 48volts, my controller says it directs 21 volts to the motor, how can I direct additional voltage and amprage directly to the motor bypassing the controller on an on and off switch where I can use it for times extra power is needed such as going up hills, I would want protection for my motor but I am not adverse to rewinding my motor in a different configeration if it helps to keep it together.