There's some evidence as it is that Muslims exercised varying degrees of control over parts of Calabria, depending on the year.
You'd need a more concerted effort to take Italy; perhaps a Rashidun/Umayyad-era spread that doesn't push into the Maghreb and instead goes across the pond. But that's harder to do since you need boats to ferry your army around. I may be wrong, but I'd think you'd have to do it before the Fatimids move their capital to Egypt; once they did that and took away the naval support out of Mahdia, probably the best you can do for Islam in Italy is surviving Kalbids of Sicily.