A bit late to the party I am afraid, but I will attempt to share some of my thoughts.
As far as I understood, OP is thinking about Kingdom of Yugoslavia, also known as "First" Yugoslavia, but its survival post-war is problematic to say the least.
Thing is, to save Royalist Yugoslavia, it has to be a much different nation then it was when it was invaded in 1941, with substantial amount of political reform in the interwar period, which would somewhat reduce the tensions between various ethnic groups, especially between Croatians and Serbs. OTL relations were poisoned at the very beginning of that nation, in December of 1918, when former Croatian members of the Austro Hungarian army protested in Zagreb, and were machine-gunned in reply. Add to that 20 years of Serbian Hegemony, brutal repression, splitting up of Croatian lands, murder of distinguished Croatian politicians and we end up were we were OTL. We end up in 1941, when Yugoslavia falls after barely 2 weeks of resistance, with 200 something fascist idiots getting in the country, declaring an "independant" Croatia and proceed to butcher anyone who is not Croatian and Catholic/Muslim.
To stop that, Croatia needs to have some sort of autonomy, such as it was given in 1939, with Cvetković-Maček Agreement, which did do a lot, but came unfortunately 20 year and too many incidents too late to do anything. Get something similar to that, and while it would not solve many of the problem plaguing the country, it would substantialy reduce the tensions between Croats and Serbs, removing much of the support far-right could draw upon, as well as for allowing for greater Royalist resistance movement to exist. Thus much of the bloodbath would be avoided, and you would have a decent alternative to the Communists, allowing for Western aligned Royalist Yugoslavia postwar.
Be aware, this is very, very basic description, but I hope someone finds this useful.