If you don't have the additional parts to make it complete the only reason that i can see that you would not get them with the new engine is if you have another engine to take them off of.
However, if they are tired parts you might want to get new parts with the new engine.
It's entirely up to you but something else to consider is if you want a professional to set up the carb, adjust the valves, install the pulse fitting, put the fuel lines on, wrap the pipe, assembly the clutch and maybe even do an initial break in and test it on the dyno.
Have you ever set up one of these carbs? Do you know how to adjust valves, Do you know if you need to install the clutch inboard or outboard?
Do you know what size sprocket you need on the clutch? Do you know if you are going to use #35 or #219 pitch chain? Is this going to be used for dirt oval or Asphalt sprint racing? Does your kart require a throttle linkage kit or not? Do you know what oil and how much you should put in the engine?
A good dealer will help you with all of this and more so you get off to a good and painless (as possible) start.