>Latios and Latias are released as roaming Pokémon in Kanto, with Latios being exclusive to SoulSilver and Latias to HeartGold. They are released by obtaining the Copycat's Poké Doll from a man in the Pokémon Fan Club in Vermillion City (you must have spoken to the Copycat in Saffron City after returning the stolen Machine Part to the Power Plant before the man can give you the doll). Upon acquiring the Poké Doll and exiting the Pokémon Fan Club, Steven Stone will appear and notify you that Latios or Latias is roaming Kanto.
-Smogon
Okay, you've done that, now all you have to do is defeat Red at the top of Mt. Silver.
Red's Pokemon are in the 80s so I suggest you prepare well before you battle him.
When Steven asks what Plate you would choose, he gives you a Hoenn starter Pokemon according to the color you picked.
Green = Treecko
Red = Torchic
Blue = Mudkip
Hope I helped!
Source:
Knowledge
