You will need to buy a new, metal gear, they don't make the plastic one anymore. The gear comes in a kit with the planetary and motor gear as well. The other gears seldom need replacement - only the main one. No harm in changing the planetary gears as well.

As Hagar said, they are pretty simple - a C clip and a pin that has to be knocked out. Be careful to note how the washers are placed so you can put them back in the same places. A little SYNTHETIC grease on the gears would be a good idea. DO NOT USE PETROLEOM BASED GREASE. It will attack the plastic ring gear.