mjm351c, Hello and welcome to the best Mustang site you could have found.
Ford never released a service part number for the ram air motors. They were part of the actuator assembly which included the motor, plastic housing and flapper door. At one time NPD sold reproduction motors so you could service the 71-73 ram air actuators. From what I understand they are no longer available and are extremely difficult to find. As Mac Posted, Perogie Enterprises in NJ serviced ram air motors and sold ram air parts at one time. You may have to call and see if they still offer that service as their web site doesn't make any reference to anything ram air that I could find.
http://www.perogie.com/ And as already posted, they can be very proud of their products and the prices reflect as such. About two years ago I found some ram air parts at Perogie's for a friend who owns a 70 Torino Cobra with a shaker ram air set up, and they defiantly knew what they were worth!
The Ford part numbers for the actuator assembly's are R/S (D3ZZ-9A626-C) and L/S (D3ZZ-9A626-D) but the chances of finding them are not very good.
There is a process where the the vacuum diaphragm is harvested from vacuum motors like you see on air cleaner duct and valve/snorkle's and modified so they may be used to repair the ram air motor. Around May of last year rackerm had an excellent write up on how to perform such surgery. https://www.7173mustangs.com/thread-hood-ram-air-housings-inserts-flap-components (See post #5)
My illustration is a original actuator which shows the vacuum motor and how it is attached to the housing which is the only way Ford serviced this unit.
Good luck with your project.