To begin, here were the original conditions of the hypothetical, as suggested by the redditor Xineph:
o Every president is in the best physical and mental condition they were ever in throughout the course of their presidency. Fatal maladies have been cured, but any lifelong conditions or chronic illnesses (e.g. FDR’s polio) remain.
o The presidents are fighting in an ovular arena 287 feet long and 180 feet wide (the dimensions of the  Roman Colosseum). The floor is concrete. Assume that weather is not a factor.
o Each president has been given one standard-issue  Gerber LHR Combat Knife
, the knife  presented to each graduate of the United States Army Special Forces Qualification Course. Assume the presidents have no training outside any combat experiences they may have had in their own lives.
o There is no penalty for avoiding combat for an extended period of time. Hiding and/or playing dead could be valid strategies, but there can be only one winner. The melee will go on as long as it needs to.
o FDR has been outfitted with a  Bound Plus H-Frame Power Wheelchair, and can travel at a maximum speed of around 11.5 MPH. The wheelchair has been customized so that he is holding his knife with his dominant hand. This is to compensate for his almost certain and immediate defeat in the face of an overwhelming disadvantage.
o Each president will be deposited in the arena regardless of their own will to fight, however, personal ethics, leadership ability, tactical expertise etc., should all be taken into account. Alliances are allowed.