[Xist]

Google before you time travel and then print the list of names and for all that is tolerable, give yourself stock picks, lottery numbers, sports scores, or something!

Patent Sildenafil in 1979. The formula is C22H30N6O4S.

It is commonly known as Viagra.

I am sure that you could find someone to help bring it to market.

Can you travel through time once or multiple times?

If you can travel multiple times you can back track their motel, but it would take a while.

Or, shortly before their flight boards, have an outside person call in and say Mohammed Atta has a bomb.

I honestly don't have any idea what the chances are that would be enough, but if it doesn't, hopefully you can go back further and try something else.