Why on earth is everybody thinking that this is a USSR wank. If anything I could see them do worse, way worse.
IOTL they suffered over 20 million casualties which was a major demographic disaster and by 44-45 the Soviet steamroller was running out of men. By the the time Soviet forces entered Berlin, man power reserves were all but gone and their economy was shattered. In this situation the USSR is likely to suffer worse (Hitler can thank Stalin for the purges, without them Germany would've been defeated way sooner).
If America is totally neutral (as unlikely as that is) it is still likely to trade with the British but less so with the Soviets. Without Lend-Lease the bombing campaign against German cities will likely be less intensive, freeing up men from air-defence roles to fight (mostly on the eastern front). Without LL the USSR will have to send men back to the farms to produce food by '42-'43 and to the factories to make trucks and locomotives, decreasing tank production. I could see Italy going down but Britain making a slightly favourable peace due to financial problems (again, if there is no LL). The Nazis and communists will eventually bleed each other white. Hitler will have to face the fact that the USSR is too big to conquer. The Soviets could get anything from a peace on 1941-borders with minor corrections or a Brest-Litovsk peace V2.0 or even slightly worse, depending on who Stalin purges for his failures and how bad those failures are.
It shouldn't be left unsaid that Stalin will likely come back for a second round though.