Why not. It seems to me true that if Kennedy had been alive, and if the election had been fixed, then Kennedy would have won. (In fact, that seems to me a likely event, given Kennedy's morals).
If you mean "successfully
fixed", then of course it is true by definition that he would have won. But if you just mean "fixed", it is far from certain that he would have done so. For example, corrupt party workers and officials could have manipulated the result in certain key states, thinking they had the election sewn up, only to find that other states had unexpectedly voted against JFK. Nothing is certain in this life. You can say he "would probably have won" if you like, but that is quite different from "would have won". Don't you agree?
And, as I said before, what if the election had not