I am guessing they didn't use a common three terminal regulator is because they run that motor below 5 volts though they could have used another type of adjustible three terminal regulator. The roof fans turn at very slow speed even with the reduction provided by the pulleys. If we knew what the voltage coming out of that board is we could better provide options. 5v from the smoke fan may be too high or they could have used a 24 volt motor driven at 5v???
Pete