The transmission plastics were never that great to begin with... I've decided not to even bother trying to use my super astute gearbox with anything more potential than a silver can 540.
Instead I've ordered the A&L belt drive transmission with a beefier slipper clutch.
There are quite a few thorp full replacement gear sets on eBay USA that would probably keep yours going for quite a long time too.
http://www.ebay.com/itm/Tamiya-Astut...QAAOSwR0JUQ9sC