In terms of drugs in general if there is a massive appetite and demand for them someone will sell it to them.
Heroin was the French Connections deal. Opium poppies were grown in Turkey or in S.E Asia(nowadays up in Afghanistan) then smuggled into Corsican labs for processing into Heroin.
Simple, another Mafia group takes up the slack of an absent FC. One of the five families in New York such as the Gambino or Genovese under the supervision of Lucky Luciano if he lives past 1962 (Genovese) and Meyer Lansky will take over.
Instead of the Poppy going into Corsican labs located Marseilles it goes to a similar manufacturing plant in Sicily then onto the U.S.