Interesting hypothetical. Since we could never get Bin Laden to sit down to a town hall discussion, his political position on abortion will always be unknown. But let’s suppose that he is against abortion for the sake of argument.
Now, the reason why people cannot vote for someone who is pro-abortion is because it is an intrinsic evil. When given a choice between someone who stands for an intrinsic evil and someone who does not, you have to go for the one who does not support an intrinsic evil. That was the case in this last election. However, you could make an argument that Bin Laden also represents an intrinsic evil given his actions and what he supports. I’m not an expert of what the Church officially labels an intrinsic evil, but I’m sure Bin Laden would fall into a few categories. If given the choice between two people who both stand for some intrinsic evil you would have to go with, as the saying says, “the lesser of two evils.” In your hypothetical, it would be Kerry.