Ref: Spectrum N Scale Train Diesel GE Dash 8-40C
What is the recommended EZ-Mate (or other brand) knuckle coupler replacement for the installed Rapido Coupler?
It depends on the version of the loco. My suggestion is to look at Micro-Trains.
http://www.micro-trains.com/conversions/sheet003.pdf
Also here - http://www.bachmanntrains.com/home-usa/board/index.php/topic,23810.0.html