Lincoln's Gettysburg Address still exists in his own handwriting. There are a very few copies, all in his handwriting that he presented to friends shortly after the November, 1863 dedication of the Cemetery... It's probable that Dr. King, Jr. wrote most of his "I have a dream" speech, although it is similar to other, previous speeches in which his writers assisted. Towards the end of the prepared text, Dr. King reverts to his Baptist preacher mode and truly inspires the crowd (and history) as though he were back in Montgomery preaching to an all black congregation...
Ted Sorenson, a relative by marriage to John F. Kennedy was a prolific speech writer (among many serving the President) and most certainly assisted in writing the many orations given by the young President...