After the split with Stalin, what was Yugoslavia's official attitude towards Soviet history before Stalin's takeover?

Tito was actually more to the right on Trotsky than Stalin, and somewhat alligned with Bukharin's position

Trotsky and Trotskyism were part of so called Left Opposition to Stalin. Although deemed as "opposition", this was actually ruling ideology in first years after October Revolution. In an essence, they called for world revolution, permanent struggle (permanent revolution), internationalism, fight against nationalism (especially Russian nationalism and "chauvinism") etc ... This fraction was often deemed Jewish, not only because of Trotsky, but also because many of Old Bolsheviks (latter suppressed by Stalin) were of Jewish ethnicity. That fact had influence on latter events, like campaign against rootless cosmopolitans etc ...

After communist revolutions after WW1 failed everywhere except in Russian Empire (now Soviet Union), this radical current lost its popularity. Soviet state apparatus wanted to strengthen and consolidate its newly acquired power, and for that they needed ideology. Stalin gave them that ideology in his Socialism in One Country. Note that he at that time aligned with Bukharin for tactical reasons to crush Trotsky, but more on that latter. According to this theory, nationalism is not so bad, each country and nation needs to have its own path to socialism at its own time. This theory actually appealed much to Yugoslav communists and Tito, because after WW2 they wanted to act independently (from Soviet Union) as much as they could.

After WW2, two things happened. First, in Europe and elsewhere in the world, Soviets started creating block of communist countries (led by Soviet Union, of course) . In order to control this block, Soviet Union somewhat moved from Socialism In One Country towards internationalism . Not so radical as Trotsky and his followers (abolishing nations altogether ), but still enough for Yugoslav communist to fill uncomfortable. Second, remaining Trotskyists moved to the West (this already happened before WW2 actually) and merged with various Cultural Marxists currents (for example Frankfurt School), forming modern left-liberal scene, especially in US. Traditionally, animosity between them and Soviet(latter Russian) state remained high till this very date, evidenced for example by current "Russian scare" in leftist media.

As for Tito and his followers, consider their local situation : high level of nationalism and patriotism because Yugoslavia, unlike other communist countries, practically liberated itself from Axis occupation, plus peasantry as large part of population (no developed industrial working class). As such, positions of Bukharin and his Right Opposition were seen as appealing, especially since after Tito-Soviet split Yugoslavia started cooperating with the West, and somewhat liberalized economy compared to other communist countries. Still, at the time this happened, Bukharin was "old news". He also had connections with former Yugoslav communist leaders like Josip Čižinski, which were removed in Stalin's purges before WW2, possibly with Tito's involvement (he was Stalin's favorite then). Therefore, Yugoslav official position on him was ambiguous, avoiding to recognize his influence on their ideology, but also sometimes mentioning him as a victim of Stalin's terror for propaganda purposes.

As for Trotsky, ideological split remained practically until the end of Yugoslavia. But again, Yugoslav communists did use his fate for anti-Soviet propaganda when opportune. Also, as Yugoslavia did portray itself as more liberal and often cooperated with the West, some contacts were established with aforementioned leftist and "progressive" circles in the West (again when beneficial for both sides) .

As for Lenin, unlike Bukharin and Trotsky, he was mentioned often and regularly. Lenin was somewhat of saint in all communist countries (perhaps because he effectively ruled only few years before his death) , so was in Yugoslavia. Here you have short clip from one Yugoslav party conference, which proclaims Tito as spiritual successor of Lenin and Marx.