@Calaca
So looking over Natsu and Ace's profile again, I have some points to say that Natsu would win eventually. Here's what I have:
1. So the fight has Natsu's best as his X792 version with Igneel's Power as his best multiplier (Seven Dragons wasn't a thing yet), therefore his power is, according to our site, "High 7-A, higher with FDKM or Dragonification, Low 6-B with DF..." as normal and "Low 6-B, higher with FDKM" with Igneel. While I will concede that Ace has the speed advantage, Natsu has more consistent upper-levels of power when, as I said before, Ace can only keep up that kind of power for a few seconds at best.
2. Natsu can eat Ace's Logia form, that's obvious and not what I'm arguing. However, once Ace discovers that, he's not going to remain in that form for his own safety. Correct me if I'm wrong, but I think Logia types can only absorb their element in their Logia forms, which means Ace can't absorb Natsu's attacks without transforming, which can give Natsu an opportunity to eat him. Natsu's bound to notice based on his past of making weakness observations on the fly.
3. Again, Natsu can obviously eat all of Ace's attacks. However, you stating that only fire with magic inside it can boost Natsu's power seems off to me. Fairy Tail describes it as just any type of fire he didn't make, with actually magic fire causing more of an effect. This site states as well, "...he can consume his particular element, except that which he has created, to replenish his energy reserves...," there's no mention of specifically Etherion-fire. I'll look further into this to double-check, but otherwise it seems that everything Ace does would power Natsu up.
4. Natsu's flames have overpowered those who have flame resistance multiple times and has gained resistance to flames that are explicitely supposed to beat his, like the God Slayer magic used by Zancrow, so an argument can be made that even if Ace was in Logia and Natsu didn't eat him, his flames should overpower Ace's like Akainu's lava.
5. On a similar lane as the previous point, Natsu's passive temperature output in his most powerful forms, like Igneel's Power or FDKM, is massive, being able to quickly evaporate a lake and instantly torch the clothes of anyone nearby. Ace may have been able to resist boiling water just fine, but I don't think he can handle temperatures that high
6. It is in character for Ace to be caught completely off guard when his powers or Logia form are circumvented in some way, like with Blackbeard or Akainu, so if Natsu started to eat him, it could be possible that Ace would be too shocked to react before it was too late.
7. The only temporary powerup in Natsu's arsenal is Igneel's Power, he can access Lightning-Fire Dragon Mode, Dragon Force, and Fire Dragon King Mode at will (Hellfire requires the Atlas Flame, Dragonification seems to have been a one time thing, Savage Dragon didn't exist yet, and he can't do Seven Dragons on his own), so he has a variety of ways to amp his power, Ace has no such options.
So those are my points so far for Natsu.