Um... If wiki is trustworthy on this, "So help me god" isn't even part of the speech, as it states in the article.
Taken straight from the website:
The swearing-in of the President of the United States occurs upon the commencement of a new term of a President of the United States. The United States Constitution mandates that the President make the following oath or affirmation before he or she can "enter on the Execution" of the office of the presidency:
I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.
The newly elected or re-elected President traditionally adds "so help me God" to the constitutionally mandated statement.
Either way, the words aren't important, it's the ideas behind them. "Don't screw this up or it will be your ass" don't sound as appropriate, neither does "I super promise to try REALLY hard!" Again, if he wishes to say it, he can. It is his right to say whatever he wants.