That's just "stuff" that you replace if you use the engine for Stunt!
Actually, to convert the Pet to CL, you just remove the spraybar, which holds the carb in place, remove the carb , and replace the spraybar...no venturi insert.
To replace the carb do just the opposite...but who'd do that!
BTW, one of the upper engines in the pic originally had one o' them carb thingies.
George