Fox's D-Air is made of loop hitboxes, meanwhile Falco's D-Air is made of normal hitboxes that don't hit the opponent repeatedly. If you remove the loop attributes from the subaction and adjust when the hitboxes should end, change the WDSK on Fox's D-Air to 0, add BKB instead and change its damage, you'll have Falco's D-Air. Tl;dr, something like this:
Fox's D-Air (vanilla):
What it should look like when edited to be Falco's D-Air:
so turn the "number of loops" and "execute loop" events into self-damage events as well, which is basically a way to disable them, then edit when the collisions terminate, which is when the hitboxes terminate. Note that Falco's D-Air changes 10 frames after it comes out, so we'd have to add space to the subaction in oder to add more stuff. For that, it's a bit more complex, so I recommend reading this:
https://smashboards.com/threads/new-melee-syntax-school-you-can-write-character-commands-now.402587/
If you have any questions, just ask.