I have a 2015 4T
Here is what I did to get 70-75km/h (roughly 40mph) depending on the wind
Installed the smooth boss and upgrade the variator to a Polini Hi-Speed (6.9 gram rollers)
Installed 1000rpm springs (Torque and Clutch)
Installed Big Bore Kit (72cc)
Up jetted the carborator to 92 (comes with 80 installed)
removed the restriction ring in the pipe between the carb and motor
Installed Silent Pro exhaust (looking to upgrade more for looks)
cut the purple/red wire on the CDI (look this up on youtube even it is there.) to take away the rev limiter
Installed new gears (15/52 to 14/53 I think. It was one tooth difference) Didn't get the extra top end above 70km/h till after I did this.
Hope this helps and shows you what kind of work goes into it. The only thing I didn't install was the BBK I let a mechanic do that.
This is all a great way to go.
A lot of my setup matches this one
I ground the collar off the variator boss on a bench grinder, made sure it slid smooth, slapped it in and it's been great since
I ran a 22mm carb, aluminum intake elbow.i have the carb, elbow, and intake runner all port matched. I can fit a quarter all the way through. I have a 72cc big bore with the big valve no emission head, a9 cam, straight pipe.
Cutting the purple wire will give you power and more top speed, however it's still rpm limited with the stock cdi. I wired in an adjustable derestricted 6 pin gy6 cdi it's easy to do.
Big bore was like 60 bucks, cam was 12, don't remember carb price. Jet kit was a great purchase. Keep sizing up til you find the most power. Cdi is 16 bucks. Get the red spring for the Trans and maybe lighter weights in variator
Cdi and carb are gonna get you the most results