This really is a go kart racing site more than anything else so we dont think in terms of Stages for parts. We buy individual parts that you wont see in kits based on what we have done in the past and on what others have done. Doesnt mean we dont know where youre coming from or want but youre likely to hear responses you dont recognize in kits. First thing is what are your budget limitations. secondly, how fast do you expect or want it to go? And what rpm range will you be riding it in most cases?
Right off the bat I would recommend a billet flywheel, a hemi piston (which is flat top) and an ARC billet rod that is .020 longer than stock. Put 26 pound springs on it with something like a Dyno CM grind cam. It will get you to a decently strong 7000 rpm. Buy a Tillotson PK1B carb, looks like the stocker but is jetted for performance and is a lot nice piece than the stocker. Easy installation. Put a decent header on it, Robertsons Torque Tubes makes a lot of pipes for karts, minibikes, or whatever. They always fit and flow good. Start there, the things I recommended are reasonably inexpensive and work well together. A set of Champion rockers will be reliable and efficient until you decide to make the Big Jump to what you would consider a stage 4. And when you decide to go bigger, leave this motor alone until youve learned more about engines and when you do jump, do it when you can afford to do it all at once- professionally ported and improved head, bigger cam, ratio rockers etc. Have fun and good luck.