The blowback chamber (also known as loading muzzle) and the plug spring.
Enhanced loading muzzles are made by both Guarder and Shooters Design. The Shooters Design one comes in a kit that also includes part # H51-23.
The spring is harder to come by - I've only seen it available in Nine Ball piston head sets - see my sales thread to see what it looks like.
If you don't mind the wait, I can bring these items in for you on my next order. PM me for details.