The priest I mentioed before had a documentary on the exorcism, and he was at the final phase in Mundelien Illinois as a seminarian. Yes it was a boy and not a girl. Fr. Martin was ordained in "43 which make a '47 exorcism possible. I ain't saying he was the exorcist, but it is possible and he is old enough.
I think there are a couple of typos in the documentary. Fr. Martin was ordained in 1954. He entered the Jesuits in 1939 was trained for 2 years as a novice and 15 years as a "formed scholastic" which is why he earned so many doctorates. I remember Fr. Martin saying in an interview, "They trained me for 17 years before I could even preach a sermon."
Fr. Martin's first exorcism was in the 1950s in Cairo, it was a possessed Muslim boy and Fr. Martin was called to fill in for the assistant who had passed out. He said, "There was simply no one else available."
(This info could plausibly have been used in the two versions of the prequel that were brought out a few years back. )
The priest who performed the exorcism in the 1940s was Fr. Joseph Bowdern assisted by Fr. William Bishop who kept a diary of the events. The diary and an account are in the book "Possessed" by Thomas Allen (it's just straight reporting)
The similarities between Fr. Martin and the fictional Fr. Marin are pretty interesting. There is debate about whether Fr. Martin or Tielhard de Chardin are the inspiration for the "archealogist" angle. Fr. Martin was definitely the more orthodox compared to Tielhard.