The simplest way would probably be to use an LM317 or equivalent linear adjustable voltage regulator. Connect the input pin to the motorcycle positive battery terminal and the ground pin to the negative battery terminal. Then set the output to 3.7V and connect the output pin to the positive connector in the camera's battery compartment and connect the negative connector in the camera's battery compartment to the LM317 ground pin. You would probably benefit from some caps at the input and output of the 317 as well.