to tear down or not

While some suggest a complete teardown & replacing everything, all the fasteners and even including bearings... I personally think this a touch overkill, particularly for such a low cost engine.

Do what you wish ofc, to each their own - but I find a compromise between that and what you've suggested to be "good enough" for these kits. I pull the cylinder, which is a mere four nuts (& perhaps a base gasket, they tear sometimes even on a new motor) - yet allows a visual inspection of all key areas (inc the amount of flash in the ports/removing if necessary), decking/lapping all cylinder gasket surfaces & cleaning the lower end of any potential debris, the only fasteners I feel must be replaced are the mount studs & nuts (too soft) & the acorn nuts used on the head studs (as they can bottom out before full head torque). The side covers you'll be pulling anyway during the build/install (clutch mag & sprocket) and I see no need to replace those fasteners as there's no load on them...

Part of the charm of such a simple two stroke is this light "top end rebuild" is quick & easy and other than a coupla fasteners won't cost you anything but a few moments of your time, yet still ensures there's no garbage in the motor and that everything's put together correctly (& w/loctite) - even if the head studs are properly seated as you'll see any discrepancy in depth (threads remaining) and length.

As for splitting the case and replacing bearings? Well... ya can if ya want to but given the generally low quality of the rest of the engine I really can't see the point aside from the amusement factor. You could replace every one yet it still won't fix the sloppy casting misalignment, out of balance flywheel & crank, sloppy wristpin & other assorted personality quirks these things are known for... you can ofc attend to those as well, but at some point you'll hafta ask yourself if it's worth the effort - or should you seek a higher quality engine? If you enjoy rebuilding for the sake of tinkering, well... that's a different story lol

Honestly? They're toys. Wonderfully fun toys but at some point disposable, no matter what you do. I've one that's got over 10,000 miles on it (all stock parts) and the cylinder's chrome liner (below ring travel) is disappearing. I could replace the piston, rings & cylinder... but why would I bother as for the effort and just a little more money I could get another that hasn't the bottom end wear? I've another engine that ate a cage at 4000 miles & instead of cracking the cases I just bought an $80 replacement motor & kept the old for parts...

Your call

My first engine out of 6 was installed right out of the box. Never again. Although I may go beyond what others feel needs to be done, I want to know that everything is good to go, weather it be 1 mile to the store or a 20 mile round trip to any one of three adjacent towns. After the first engine was installed (cylinder studs not torqued down, blew a head gasket), I have never had trouble with the remaining 5. Here is what I now do to all my engines, pick what you like, discard the rest, but keep it handy.

https://docs.google.com/document/d/1GcesQihc7-mm3hJGf7UPiqOaRXZ8ekUT99QUZq2qXXE/edit

Should you have starting problems, take a look at this:
https://docs.google.com/document/d/1omvHOrPyn5zvv-2sx-GNyWPjmryEeLt7zYVQup22ppE/edit

The MOST important thing to remember...install the engine correctly..make sure the mounts are as flush with the tubes as you can. The optimum "V" angle is 75 degrees.

I've worked on all kinds of engines in the past 50 years, these are the simplest to work on bar none.

i have run mine right out of the box,no problems at all,other then idle needed adjusted,chain adjustment,clutch arm adjustment just the basics,nothing internal.now maybe ive been lucky,dono, only two builds.one from bgf other from pirate cycles.third build still in box,grubee starfire,nos.gt-3. though it wouldnt hurt to do visual inspection of motor,clutch,magneto, clean fuel tank etc.

I bought my own engine from That'sDax years ago. At that time, I'd read that he had a good reputation for customer service in many people's opinion. So I did not feel a complete tear-down was absolutely necessary.
But I did check for lubrication in the clutch actuation, checked head bolts and case bolts and carburettor assembly. It all seemed to have either been gone-over before or assembled with some care. This same engine survived a wreck that the original bike it was on did not. It is now mounted in my current bike and running fine, so I am resisting the temptation to muck about with it too much.