Ace may have held his footing against Aokiji at Marineford, particularly by melting the latter's pheasant beak, but that doesn't imply he (Ace) would have been able to beat him (Aokiji). Aokiji is still an Admiral-level opponent, was an Admiral once, and is leagues above Ace. Even if we assume that Ace improved greatly over time (which there is no real evidence for), it still wouldn't have been enough to defeat Aokiji. There were also two other Admiraals present at the battle of Marineford: Ginryuu and Gudeneyuu. Neither of them are listed as being beaten by Ace, but they both clearly respected him so much that they gave him their own swords after he lost his own. If anything, this shows that even an Admiral-level fighter like Ace could still have a hard time defeating Aokiji. In conclusion, yes, Ace can beat Aokiji. However, it wasn't because he was an amazing fighter, but because Aokiji was too weak willed. If Aokiji had been willing to listen to reason instead of trying to fight to the death with Ace, then he would have been defeated easily. As it stands, he was only able to resist for as long as he did because he thought he could win regardless of how many wounds he took from Ace's attacks.