There are many variables to this, the input air temperature, the motor loading and efficiency, air density at the altitude installed,use cycle, ie 100% or as low as 25% use.
If you are trying to fit an electric cooling fan and ducting I would try to establish the original design airflow by running the motor at it's rated speed and measuring the airflow. The electric fan and duct would be designed to achieve these air flows and a motor mounted thermostat would control the fan