You can replace the bearing to save the most money. You will need to buy the bearings and use a bearing puller. But if you want to replace the idle pulleys, there are three in total. Use the guide from pelicans.
https://www.pelicanparts.com/techarticles/Boxster_Tech/17-ENGINE-Idler_Pullies/17-ENGINE-Idler_Pullies.htm
If you want to replace the bearing only, Ben Burner on Youtube has an excellent video on how to do it. To find the model # to replace the bearing, you should take out the idle pulleys to confirm the right part.
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