I think that having the USSR begin life larger might help.
I don't know if there's a logical way to have the USSR keep Russian imperial territory (Poland, Finland, Tuvan Republic, Moldova, Baltic states) but that would likely be a help. More so if they can grab any territory from Turkey/Germany/AH.
Nextly when Mongolia gets it's independence, having Inner Mongolia come with it would help seeing as it ended up as a Soviet puppet state.
With this larger USSR, Stalin's invasion of East Turkestan in the 30s might actually succeed in fully annexing the territory.
This of course leads to a smaller and less powerful PRC which can probably be twiddled to avoid the Sino-Soviet split.
IMHO, if this is possible, then barring some catastrophe, the USSR still stands to this day