it's not that the airport tarmac was damaged by the back-to-back hurricanes. The runways are fine. It’s that the entire electronics system for running a major airport — lights, radar, computer systems and so on — is down, causing the cancellation of hundreds of flights.
Again, the U.S. military has equipment that could fix this quickly. The Pentagon can airlift the materials and supplies needed to establish a high-volume airfield operation for military actions anywhere in the world, and on short notice. That equipment — mobile radar, air traffic control systems and lighting — could be brought to San Juan immediately by air and operations could be set up using military air traffic control personnel who know how to run it. The U.S. military set up a large operational airbase at Kobani, Syria, within a matter of days in 2015, taking over an abandoned airstrip there and airlifting in all the necessary supplies. If it can do that in a combat zone under primitive conditions, it could certainly return San Juan International Airport to functionality in similar time -- if ordered to do so.