Instructions
Go to the Power Plant by using Surf to cross the river just above the northern entrance to the Rock Tunnel. Speak to the workers inside to learn about the theft of the missing part of the generator. Try to leave, at which point you'll overhear that the thief was spotted in Cerulean City.
Head to Cerulean City and attempt to enter the gym -- you'll run into a member of Team Rocket fleeing from the gym. Go to the bridge north of town to find him, then engage him and his level 39 Golbat in battle -- after defeating them, the Rocket will tell you that he hid the machine part in the gym.
Go to the Cerulean Gym and press the "A" button as you move around the buoy in the upper left of the gym, near the guard tower. Obtain the machine part and head back to the power plant. Give the machine part to the head technician to restore power to the region.
Go to Lavender Town and enter the radio tower. Speak to the man near the stairs to receive the expansion card for your Pokegear, which will allow you to listen to the Pokeflute station -- you can wake Snorlax by turning on your radio, setting it to the Pokeflute station, and pressing the "A" button to engage Snorlax.