Why not go through all scenarios?
Scenario 1:The US ramps up its industry despite not actively participating in the war, US gives LL, American merchant ships with American crews, American warships for escort and even personell to build the Iranian route - basically everything short of hot war. As said before, its not very realistic that the US shows this kind of involvement and isnt dragged into the war, but oh well. In this scenario the British and Soviets are supplied with the same amount of food, resources and tanks, aircraft, trucks as OTL, but have no American troops and air forces helping them. This means:
The NA campaign doesnt change much in 1941/42, the British need a few months longer to win in NA. However without the US air force, Germany sustains much less aircraft casualties and might even defeat the British Bomber Offensive in 1944; additionally it sustains less damage to its cities/industry. While the British might invade Sicily and slowly advance through Italy, they have no way of invading Western Europe.
Meanwhile the Soviets are faced with a lot more German aircraft and other equipment due to less damage to German industry and with a lot more infantry due to the absence of American troops, so their advance is slower. If the US continues to supply the USSR/UK with LL even after Roosevelts death in April 1945, then the war might end in 1946 with the Soviets on the Rhine. If the US stops giving LL after Roosevelt is replaced by Truman, the Germans get a stalemate.
Scenario 2:The US ramps up its industry despite not actively participating in the war, US gives LL,but no American merchant ships with American crews, no American warships for escort and no personell to build the Iranian route. This means the British get most of their OTL LL food and resource supply, the Soviets get far less (basically what the British can supply through the Northern Route). The British need untill the end of 1943 to clear North Africa and they might invade Sicily in 1944. If they invade Italy the Germans can contain them. The Soviets are stretched to their limit; their advance is much slower than OTL. In this scenario a German defeat/stalemate is dependent on more than Roosevelts death and can lead to total defeat or stalemate by 1947.
Scenario 3: The US doesnt implement LL and stops Cash&Carry after the British run out of cash. That means no US support at all. Britain starves and compromises by 1942/43, the Soviets starve and compromise by 1943/44, Germany wins the war by 1944 or gets a very favourable stalemate.
Any objections?