Off the top of my head, taking Bolshevik leadership out would do the trick. I have a gnawing suspicion that, after Kornilov's death, any "White Victory" TL would produce a rather weak Russia. And Kornilov surviving would be, well, interesting...
It would be folly to suggest a POD to bring Bolsheviks down without ASBs opening safari on them. Bolshevik's situation in 1918-1919 is rather like Russian Empire in 1916-early 1917. Regime seems to be ever-strengthening on the surface, but very weak-legged and liable to fall with big bang once pushed over invisible fault line. And this line could be any. Lenin killed, Yudenitch's forces taking Petrograd, some mad dash of Shkuro's cossacks toward Moscow, Pepelyaev taking Perm.