'Legal' and 'Moral' are different things. Legally, only the Base Stat Total and stuff like illegal moves matter. Morally? Most people oppose hacking and see it as cheating.
I take a middle ground. I had my Gardevoir and Latias hacked to be shiny, and Kyurem turned 6 IV, with the two dragons having fixes to their natured.
Legally, they're 100% possible in game without hacking, but morally? Most people get irked when I mention I got them hacked.
So it's rather opinionated. I wouldn't hack event legends... As this makes them illegal unless the original even was for a shiny anyway.