In case you haven't done so yet, go to the Power Plant and talk to the Director. Then talk to everyone else. Then, go to Cerulean City and there should be a Rocket Grunt there. Let him run away. Then, go towards where Bill's cottage is. (North of town) On your way up you will see the grunt again. Fight him. When you beat him, he will tell you he hid the missing part in the Cerulean Gym. Surf on the little square of water in the middle and press A while facing the top left part of it. The missing part should be there. Go to the Power Plant and give the part to the director.
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In Cerulean gym
the director will give u after giving the missing generator part to the manager . he will be in the radio tower of levender town
in the gym in cearulean city
After meeting the man in the powerplant, go to cerulean gym there you wil find a rocket grunt after beating him, find the missing part in the gym, it in the water.
In Pokemon Crystal.