I don't think Italy needs to be divided in too many kingdoms. Since JD already broke Sardinia off (though it could be further divided into a Duchy of Corsica and a Duchy of Sardinia proper), and the southern half is already a different kingdom (Sicily, which gets further divided de-jure and de-facto in the last startdate into Naples and Trincaria), I think the remaining kingdom should be divided into two, Lombardy (k_lombardy, which is already in the game, just unused), which would be the northern half, and Italy itself, in the central portion. The border would follow the duchies of Tuscany and Romagna, which would be in Italy while everything north of them is Lombardy. Venice can stay as it is, a one-province Kingdom, and you can also make the Papacy de-jure Latium.
Only think to note, would be to also move Italy's capital to Ravenna (it's currently Padua), and to remove the Lombard kings from k_italy and place them in k_lombardy history, and fix history file accordingly.