My gear suddenly broke down when going at high speed: Stuck in one gear, lots of noise and much resistance but could still use the bike, and I noticed plastic cogwheels missing teeth on rear derailleur.

I happened to be very close to my city's biggest bike shop, so I went in there and they were nice enough to say I need new gear (rear derailleur) and the chain is worn out but they called the gear "antique" and said they had no such old gears.

I also haven't found it on the internet but some speak of a Shimano "Rear Derailleur Adapter"*. Is that what I need or how can I fix this? The bike is not that old, maybe 10 years. Hope you can help. Took a pic of the wreck:

Cut.jpg

Cut2.jpg

Cut3.jpg

*) Not allowed to post links but here's one: cheapbikeparts360[dot]com/products/shimano-sis-rear-derailleur-adapter/