None of the continental Muslim enclaves had the substantive backing of any major Muslim state. Fraxinet, the Emirate of Bari, and (probably) the fortress on the Garigliano were all essentially set up by "adventurers." They were a phenomenon of late/post-Carolingian weakness, and could only thrive in a vacuum of power. As soon as a local power of significance arose and was sufficiently motivated to take an interest, each of those enclaves was wiped out.
So to manage this, you almost certainly need a Muslim power in Sicily/Tunisia with real manpower. That's not difficult - the hard part is getting them focused on Italy. The Fatimids, for instance, were certainly powerful enough - they raided Italy, decisively defeated the Byzantine army and navy in several engagements, and were able to lay siege to (but did not take) significant cities like Naples. But ultimately their interests did not really lie in Italy - they wanted Egypt, which was a much richer prize (and already majority Muslim). Once they had Egypt, Tunisia became the periphery, and it eventually slipped away from them.
That's the paradox - to invade Italy, you need a strong Muslim state in Tunisia, but a strong state in Tunisia is going to be naturally more interested in expansion in Africa - west into Morocco or east into Egypt - than in the conquest of the little principalities of southern/central Italy, whose territory was not particularly rich or attractive in comparison. Once they have Morocco or Egypt, they're going to become embroiled in the politics of Andalusia or the Levant (respectively), and Italy gets put on the back-burner. It's hard for me to see a way out of that motivation problem.