Here is how the editors of book preface the letter (my notes in blue):
Joe Pasternak, a producer of 83 movies, wrote Reagan that Universal Pictures once vetoed his decision to cast Reagan in a Deanna Durbin movie. Durbin had starred in 23 movies (actually, 21) in the 1930s and 1940s. Pasternak jokingly added that if Reagan had been given the role, he would have become "a bigger star and our country might have lost a great president."
Unfortunately, it wasn't specified which movie Pasternak wanted him for. Reagan responded to the letter as follows:
Mr. Joe Pasternak
Beverly Hills, California
July 22, 1987
Thanks for your letter and for that job of no casting you did back there at Universal. I probably wouldn't have thanked you if I'd known at the time but now that I'm in an eight-year run of the play deal, things do look a little different. Of course I'll be at liberty in about a year and a half. Maybe I could play the life story of Mickey Rooney, or am I too tall?
Seriously, it was good to hear from you and I thank you for your generous words.