Well, I don't know a lot of specific details of 9/11 despite a being a NY tour guide (I work mainly in other areas than the Financial District), but I have read one or two books on the subject. Though it's been years I do remember the basic, but very interesting fact that John O'Neill, an FBI agent who heavily investigated Osama Bin Laden while in the agency, in 2001 was actually the chief of security for the World Trade Center. Honestly I think if I just went down to the WTC and mentioned him by name with enough details to create urgency I imagine that despite being retired from the FBI he would both have the motive and ability to look into it and prevent the attacks. Hell, his old workplace, FBI's New York City's location would only be a ten minute walk from the WTC.
As for waking up naked in Central Park I imagine it depends on where I woke up and what time it was, but I would probably try to make my way to the Delacorte Theater and hop the fence to see if there were costumes I could steal. Even if it was an elaborate Shakespearean costume it wouldn't matter since really I just need something to wear to avoid being taken away for public indecency.
That might fail for a number of reasons though (security, might not be any costumes.) If that was the case I would run into the Ramble where naked men traditionally have had places to hide and hang out in the bushes till I saw somebody I thought I could get clothes off of one way or the other.
If that failed honestly the worst case scenario is I get arrested for public indecency and have to mention John O'Neill plus a possible terrorist threat at the police station. As long as I leave the whole time travel thing out and don't look too crazy I could probably get in touch with him eventually.
Anyways that's what I would do. If you want to learn more about the attacks and John O'Neill I recommend The Looming Tower: Al-Qaeda and the Road to 9/11. It should also be mentioned that John O'Neill would himself lose his life during the attacks on the World Trade Center.