There are some figures on what % of Lend Lease aid to Russia was lost during delivery. For example, according to the US Dept of State "Report on War Aid" figures, some 14747 softskin vehicles were lost en route, which is a pretty low % of the total delivered, less than 5% actually. The vast majority of the aid appeared to have gotten through.
The other thing to realize is where the aid went and why it was so critical. It kept the Soviet logistical infrastructure in operation. Sure the Russians made their own planes and tanks in the thousands, but what's the point if the Red Army had no trucks or locomotives to deliver them and other vitals such as food, ammunition, spare parts to the front, never mind transporting the troops themselves. If the Red Army Rifle Divisions have far fewer trucks how are they supposed to get around? Walk? Remember that after the first year of war most of the horses are already dead.
Without Lend Lease there would have been around 430000 fewer trucks available to the Red Army and NO Soviet railroad system. Lend Lease was utterly decisive in the Red Army being victorious over the Wehrmacht.
Put it this way.
No Lend Lease, no Soviet victory.