It looks like this narc is entirely scripts. By changing these scripts you change what Pokemon appears.
The game calls a script, say when encountering Terrakion. It loads up 7_458, and the Pokemon that is scripted to appear is Terrakion. Changing the hex Terrakion is present at (78 01 7F 02 2A) allows you to change what appears.
I have no experience scripting, but all I can say is that there is absolutely no relation structure wise compared to Encounter Slots and trpoke/trdata. Those are a set size with easily interpretable meaning, scripts are not a rigid size/structure!