I had a polini carb kit from ScooterSwapShop but it would always have a quick throttle bog no matter how i tuned it for the 2 years i had the kit.

This is the setup i run now and i love it, it put on without tuning already ran amazing and definitly felt powerful with no bogs at all once setup.

you need to get bolts to hold the adapter to the intake manifold, also put the Permatex blue gasket maker inbetween the manifold and adapter. you also need to route the thottle cable a different way above the box.  i prefer the hose style mount since you can pop the carb off and switch jets with a single hose clamp.

This is the exact same as the other carb it except its bolt on carb with a built in flange so you dont need to buy an adapter aswell. This makes it impossible for the carb to fall off but also makes it much harder to take off, tune, swap jets, and IMO i prefer the hose style mount since you can pop the carb off and switch jets with a single hose clamp.

The Yumanishi 125cc big bore kit increases the displacement, compression, lighter piston, stiffer valve springs and retainers for potentially higher rpms, and  a better higher lift camshaft

This is the first big bore kit avaliable for the Honda Navi. To do this you will be required to either get someone knowledgable with small engines to dissasemble and reassemble the engine for you or be able to yourself. 

You also will need to find a local machine shop that is able to bore the stock cylinder jug out from 50mm to 53mm.

You might have to either bend or cut and weld the header but ive heard a ton of people also say it bolts right on but is a tight fit. once thats done youll need to modify the included bracket or make your own, its not hard to do. Then just pick any 51mm slip on and it should fit the header, all in I made a more "premium" exhaust setup for my navi on aliexpress for roughly $150-$170 canadian shipped........ for comparison reckleass resells gy6 exhausts that are the exact same for $300 usd not including shipping and just a custom bracket which IMO isnt worth that insanely steep mark up.  

This is an all in one kit for bar replacements. I love the look of these bars on the navi, i mainly got them to get me more into a tucked positon but they ended up looking really good visually. only downsides are you need to find your own bolt to use the mounts and the included mount are low enough they make the handle bar lock not usable but if you got bar risers it would work fine. it also includes the dirt bike foam bar piece with a pro taper cover which i like the look of with my bike, also made a great place to mount my tach. Last thing is the grips which tie in the whole package looks wise and all around everything matches super well and fits the color of the navi amazingly.

The biggest improvement for the money on both acceleration and top speed comes down to cvt tuuning and its setup. First focus on the CVT weights, you can choose from both rollers or sliders of various weights but they have to be 20x15 to fit the OEM variator. Most commonly people settle on weights in the 11g-13g range.  I personally run 11g sliders but i am a very light person so your best weight could change based on your own body weight and if you have a ton of hills where you live. 

I found in my testing the OEM variator does best top speed wise in the navi aswell as the OEM belt. Some people run a longer belt but i found it to be more of a band aid overtop of poor cvt tuning at least in my setup it didnt help.

After tuning the variator weights perfectly move onto upgrading your clutch and clutch bell, any GY6 125cc clutch and bell should work and you can even find package deals for both pieces.

lastly if you want to squeeze just a little more out of your CVT you can try the variator mod i made a video on. it seemed to have helped me and a few others slightly and it isnt as hard of a mod to do as you think. ive ran my navi with the modified variator for 6k kms now without issue.