Let's say that for whatever reason they are able to stay neutral indefinitely (Stalin's able to convince Mussolini that his rule is consistent with fascist principles, for instance, and Mussolini convinces Hitler to focus on consolidating western Europe first). Can the Allies still win without Soviet support?
Neutral Soviet Union..well, it will have 4 republic less (Latvia, Lithuania, Estonia, Moldova) and Belarus + North-west Russia will be a bit reduced without all the conquests of 1939-1940 made in alliance with Germany.
In this case, Germany has much reduced motivation to attack Soviet Union too, and their primary focus of expansion may shift to England. Not sure it will come to actual invasion attempt or not. May be some sort of ceasefire will be reached by 1943 after several inconclusive naval encounters and bombing.
Also, this scenario mean harder African and Pacific theaters of WWII, because forces IOTL spent against Soviet Union can be pumped to another location. But the result is still the same. Just force ratio changes from 5:1 IOTL to 3.3:1.