Just want to note the legality issues with Tamamushi Magikarp, the event you're on about.
Japanese Gen 1 and 2 games cannot trade with international games, this also goes for Korean games. Pokemon Showdown simulates English Yellow and Crystal, so it cannot be used there. This is also an issue with Fly Pikachu, Pay Day Rapidash and Pay Day Fearow. It was allowed for a time, but only because this fact wasn't known.
If you want to use it, I believe it can be unbanned through challenge commands, or you can just use custom game.