i'm gonna take a guess at what the shop is actually talking about .. There's a set of "sprockets" that slipped my mind.. The chain tensioner assembly.
it's got two, little plastic sprockets, very similar to a 10-speed bicycle derailleur chain tensioner. No bearinga to speak of.. they just turn on their shafts.
The weak sprocket support gets tweaked easily, after which it's almost impossible to get lined up properly. The sprockets can then snap off if you pedal hard or pedal backwards, and you're not gonna be able to pedal-start without it.
I almost tried making one myself since they are so simple, but found a spare. Same thing may have occured to the mechanic in the shop. Replacing one of these plastic sprockets with brass would make sense.
If the shop is refering to this chain tensioner, Ebay has one