This discussion mainly focuses on lend-lease, and that was of course the question asked. But the premise given to arrive at a hypothetical situation, was:
Had Murmansk been captured, Japan went to war with U.S.S.R and capture Vladivostok and the Germans captured Stalingrad, could the U.S.S.R survived without outside help?
So we're not just
talking about a blockade to most of the land-lease supplies that would reach the Soviet Union. In this scenario, there's also a Japanese force fighting the USSR in the east, the Germans would actually have captured Stalingrad (which they did not do in reality - they captured most of it but the fight in that city never stopped), and the with Murmansk in Axis hands, German and Finnish troops from the far north could have wheeled in to cut the supply lines that Leningrad so desperately needed.
I think those strategic factors would be at least as important as the loss of the lend-lease supplies as such.