Cook noodles according to package directions.
Heat oil in a large cast-iron skillet over high. Add scallions and shrimp. Cook, stirring often, until scallions are bright green, 1 to 2 minutes. Add egg; cook, stirring constantly, until egg and shrimp are cooked, about 1 minute. Add noodles, half of the peanuts, ketchup, lime juice, Sriracha, fish sauce, and 1 tablespoon water. Toss well to combine. Serve in shallow bowls; sprinkle with cilantro and remaining peanuts.