How to write a speech on "Inaugural Address by John F Kennedy" when it already is a speech?
One approach on writing a speech about a speech is what newcasters do after the President addresses the Nation: They comment on the speech and its salient points and analyze it as to its effectiveness, etc.
With regard to the inaugural speech of JFK, you could analyze/comment upon Kennedy's use of rhetoric; that is, explain how Kennedy's words influenced the thoughts of his listeners through phrasing, figurative language, emotional appeals, etc. Afterall, Kennedy's speech, like the "I Have a Dream" of Martin Luther King and the "Friends, Romans, Countrymen" speech of Marc Antony contains many moving emotional appeals, the most famous of which is "Ask not what your country can do for you; ask what you can do for your country."
So....research rhetorical devices and see how they apply to this speech if you wish to take the analytical approach.