Tried to modify the "refueling_time_factor" in Couatladdons.ini file but makes no difference.
That setting is no longer used, which is why you cannot access it anymore from the UI. The speed of the refueling process is controlled by the performance of the fuel truck pump, which changes depending on the truck used, but use real world specs.
Since the fuel trucks supplied with GSX varies between 500 gpm and 1000 gpm, and the C47 can hold 900 USGAL in the internal tanks + 900 in the cabin, 2 minutes seems to be a correct time, depending how many tanks were modeled in the plane you use.
How do I adjust the refueling rate?
You can only speed up, not down.