Avoid the destitution of Mussolini on the 25th of July 1943 and you will avoid the formation of (as big as OTL)
Resistenza partisan insurgence. With hard liner fascists still in power there will also probably not be anything short of unconditional surrender and therefore no cobelligerance period. This will probably mean that Italy loses more on the eastern border (in such a scenario Trieste may remain a "Free City" or be included into Jugoslavia) and maybe there is a temporary French occupation zone in north western Italy (if you are willing to stretch things the Valle d'Aosta could go to France). There were also the talks of Sicily as an american state, but that would be ASB, although an independent Sicily might not be off the charts (would turn into a nice banana republic...).
Socially and Politically you would probably see more something akin to de-Nazification than what happened in Japan, because Italy still came from (in western eyes at least) a more solid history of constitutional government, closer to Germany than Japan.
But, how to accomplish all this? You have got to make the Fascist regime become more totalitarian and idealy to get rid of the King, or at least to make the Royal Family totally powerless, which is rather difficult without giant butterflies forming. A Fascist Party where the likes of Starace and Farinacci have even more influence might become more similar to what the National Socialists were doing in Germany, but it is not easy, because Mussolini was in the end an opportunist without a totally clear ideological base and because of the moderating influence of the Roman Catholic Church. On the other hand, if you somehow get the Futurist movement (there was a Futurist Political Party founded by Marinetti in 1918, but it became irrelevant.
Here is its manifesto, unfortunately only in Italian and notable for its extreme anticlericalism, among other things) into political power, you could have
interesting evolutions in Italian society, which would necessitate very deep reconstruction after the inevitable defeat in a WW2 equivalent.
EDIT: As to the impact of a Japanese solution to Italy (although it is imho implausible to transplant to Italy what happened to Japan, you would probably see on one hand a return to traditional Catholic values (especially if the alt-fascist regime had been strongly anticlerical), but on the other hand an even more thorough acceptation of "American" culture and lifestyle, with a significant suppression of communist and socialist parties. The economical evolution will probably be similar to OTL, with Italy using its comparatively cheaper manpower resources to have something similar to OTL's economical boom in the 50's and especially the 60's.The main difference will imho be in the names of the enterpreneurs, with the Agnelli family (FIAT owners) either purged by the regime or after for beng collaborators. Probably less State owned companies and an earlier influx of American (French in their occupation zone) companies controlling key industries (electricity, metallurgy, TLC etc). If the French occupation zone last long and extends to the whole of Piedmont and Liguria, the rest of Italy has lost much of its industrial heartland and this might hamper growth.