The Russians are seriously effected by the lack of lend-lease as they actually had less of a production capacity than the Germans. They were able to focus on several things and mass produce, which increased the number of items produced. If they need to manufacture large numbers of things they in OTL did not need to make, then this benefit drops significantly. Also, the Russians had great difficulty making high tech manufactured goods like radios, which would be crippling. One of their greatest handicaps during the first few years of war was that vehicles (tanks) could not communicate with one another and the Germans would out maneuver and destroy them. Even the vaunted t34 was far less useful without this technology. Also very large amounts of winter gear were shipped, which means that Soviet soldiers might be freezing after the first winter. High grade aviation fuel was unavailable to the Soviets, but they obtained it in large quantities from the US, which means that their airforce might have had an even greater reduction in effectiveness.
The fact of the matter is that without US goods, the Soviets are screwed. They would not lose the war (baring exceptional German leadership that was lacking), but they could not win it. I really do see a Brest-Litovsk peace, with both sides hurt and getting ready for the next round. However the Russians would be much worse off, more losses than OTL with none of the gains.