Given the complexities of the various RNG systems for Gen 3 events, such as restricted seeds, OTG or SID being part of the generation procedure etc,
I recommend against editing PID/IV/Nature/OTG/ability/SID any 3rd gen event.
Hence, no legality guide will be linked (not sure if one even exists for those)
If you're importing such a mon into one of the later gens, there isn't a real need to get flawless IVs etc.
Gen 6: Ability capsule, can change the mon's ability, from (1) to (2) [and vice versa] if those abilities differ
Gen 7: ^ + Hyper Training can max out IVs (they'll appear to have flawless IVs, and will function that way in battle).
Gen 8: ^ + mints to change stat nature. (edit: and if the leaked content relating to Crown Tundra stays the same [GF added some CT content in the IoA update; for abilities, you can also 1/2 -> HA)