Hmm. I'm not sure how much of a "challenge" this really is. It seems pretty straightforward to get it happening...
A group of southern white racists decide to kill Kennedy, research his upcoming movements, scope out some good vantage points, and send someone in to blow his head off. They could probably have carried it out in Dallas on Nov. 22 1963, just like Oswald did. Though no reason why they'd have to pick that particular venue.
If you wanna get more into the logistics of it, I guess the Martin Luther King assassination a few years later might provide something of a reverse-time template. From what I've been able to glean, it was general knowledge in the southern criminal underground that certain wealthy racists were offering a bounty on anyone who took out MLK, and James Earl Ray figured he'd do the deed and collect. I'm not sure if he ever did get any cash(his subsequent jet-setting might seem to indicate he did), but in any case, someone could try the same thing with Kennedy.
Mind you, putting a bounty out on the POTUS might get you on the federal law enforcement radar a lot quicker than a bounty on some rabble-rousing "negro communist" preacher would. So, any rich southerner with a sense of his own self-interest might want to think twice about spreading the word that he wants Kennedy dead. More likely, the assassins would just be devout klansmen, doing it pro bono for the supposed public good.