Can a motorized damper be installed inside existing ductwork?
Try looking up belimo motorized dampers. They are excellent dampers, hopefully they are available in your area. 

This is a Belimo damper actuatorBelimo 

This is just an example. They do have less expensive ones. You'll need get a sheet metal shop to make the damper for you. You need to be very specific about the sizes and dimensions. If you don't use the right terminology they may get upset. You will probably need to use whatever damper actuators they stock.