I think the urban issue is important in the context of what it was to be a maritime Republic. They were city-states. So a Sicilian Republic would more likely be a Palermitan Republic, or possibly Catanese, Messinese or Syracusan Republic. Unfortunately non of those cities really developed an urban civic leadership because the whole island was always under someone's control. Where as northern Italy fragmented and allowed civic power centers to develop.
Furthermore the Sicilians had a long tradition of inviting in people to take over rather than deposing rulers themselves. Starting with the Greeks Romans and Carthaginians. Then the Arabs were invited in to get rid of the Byzantines, the Normans to get rid of the Arabs, Aragon to get rid of the French, the Savoys were almost invited in the get rid of the Bourbons but in the end the job went to Garibaldi, the Sicilians, especially the mafiosi, cooperated with the Allies to get rid of the Fascists. I'm simplifying things a bit of course but still the broad outline remains. Perhaps because it was such a cross roads or perhaps it was some quirk of fate.
I suppose it might be possible with a POD before the Emirate is established. If say the Byzantines remained de jure in control but their power broke down like in the rest of southern Italy. The Saracens begin raiding the island like they attacked the Italian mainland and then the locals start to defend themselves. Palermo could potentially emerge as a local power center to rival the Byzantine center of government in Syracuse. Eventually the Palermitan Republic takes control of the whole island as the Byzantine Empire withdraws. It participates in the expeditions to Sardinia and emerges as a Maritime power.
But after the Emirate is established its rather difficult. Because someone has to kick the Arabs out and its not going to be the Sicilians. A general uprising against the Arabs the results in a Republic is unlikely. And there's no reason for a conquering power to set up a Republic in Sicily after they take over the island.