The main problems that caused the rise of fascism would still be there (i.e. fear of communism, political instability, delusive outcome of WWI), so I don't think Mussolini would give up. They might follow a path of getting more and more seats in parliament during multiple elections, like the Nazis IOTL, and they would probably still gain power by the end of the 1920's. The left was too divided internally to ever pose a threat to their ascent to power.
Butterflies: probably Hitler wouldn't be inspired to try the Beer Hall Putsch, thus no prison and likely no Mein Kampf (but that doesn't mean no antisemitism). They would probably still ascend to power like IOTL, because their ascent was triggered more by the 1929 crisis.
Maybe, without the fascists in power at the right time, Fiume might remain an independent state under D'Annunzio, or anyway it wouldn't go to Italy. Also, Matteotti could live, at least a little bit longer. He might still denounce the Fascists later, but given that his OTL murder could have been a misinterpretation of Mussolini's words, it could be butterflied away.
In conclusion, I don't see any major changes if not for a later fascism in Italy