It would have been highly unlikely that a libertarian-influenced foreign policy of cultural exchange and trade with people of other nations (note that I wrote "with people", not "with nations")would have resulted in attacks on the WTC and Pentagon. If it had occurred anyway, then it would have been treated as a criminal action and handled accordingly. Intelligence ties and cooperation with other nations would have been used to identify and arrest those responsible. Though libertarianism opposes the initiation of aggression, it is not pacifistic. So, if a country would not turn over those identified as being responsible for such an attack and other attacks are anticipated, covert or overt military or paramilitary methods for bringing an end to such a thread would not be rejected.

Subsidies to businesses would cease, as would subsidies to many other entities (e.g., state and local governments, favored special interest groups, alternative energy hustles, etc.)

Such police-state entities like Homeland Security and other intrusive and suppressive policing actions meant to increase control over the citizenry would cease to exist under a libertarian-influenced society because there would be no government-caused need for it and the government would be too small for other power structures to enlist it for its own benefit(e.g., corporations).
"When all has been said that can be said, and all has been done that can be done, there will be poetry";-) -- Issodhos