Tambo ......... one doesn't "throw out" an Aga ............. one puts an ad in the local paper and invites someone to take it away whilst crossing your palm with muchas gelt ;o)
Anyway, Sad OG........ with a standard 16" fireback, I guess you have a regular 7" clay flue. Once it's had a good sweep, an adaptor could be fitted that connects to a w/burner flue.
There are many ways of siting the burner relative to the opening.... set in to it, or sitting in front of a closed opening........ or anything in between.
Check the local press for "flue & chimney" specialists in your area, or maybe a burner supplier. They will visit and assess it for you without obligation.
Over a certain kilowatt output, an air supply would be needed, but that's another story.