NY is digitally created so not the hard. They wouldn't necessarily need to impose the new host into the ride footage. Make it "Soaring over NY" with a few changes - specifically on the last 1/3 of the ride. The ride isn't overtly Fallon til we reach that point. You can still race through traffic, race through the harbor, see the Statue of Liberty. Maybe add Time Square, Central Park...
If they went the classic NBC shows route - create an NBC-fictitious New York City. Add the Cheers bar, Dunder Mifflin offices, the Friends' apartments, some SNL sets...
If they can turn Earthquake into Disaster into 2 months - I wouldn't be surprised if a change would take a 3-4-month turnaround.