Can't Ace just put out the flames on King's back by absorbing it? This would be a great counter to his durability, and I believe King's Speed Amp isn't that much of a problem, as Ace has already reacted to attacks from Whitebeard, the admirals, can also boost his own speed with fire and has far better precog.
Ace's flames are so hot that it even burned Whitebeard's skin, who has an absurd resistance to heat, I don't know if King could handle them. King's advantage would be at close range, as Ace doesn't master physical combat, but his feats, like keeping up with Yamato and Jinbe (two natural fighters at close range as well) don't allow me to assume he'd be stomped in that regard.
Area of effect is an absurd advantage for Ace whether with dragons, Hiken or Entei, things Ace can spam (remember the first Hiken Ace launched in the series, so vast it swallowed multiple ships easily). Frankly I think Ace wins but King is a better fighter I would say. Ace likely wins with high difficulty.