I think this kind of counterfactual is pretty impossible to do.
Do you mean that it would have been won without any US involvement? Or do you mean the US involvement would have proceeded similarly with a different president?
From my cursory sense of things it seems like both are probably true. The U.S with FDR obviously contributed to it ending when it did, but so did other countries who were there earlier and sacrificed more. The U.S seems like more of a winner in the sense that they sacrificed relatively little while getting the most out of it, but I don't know if that's a good use of the term "won" in the context of a world war.
So Stalin may have played a part in ending WW2, but don't forget his part in starting it.