I think it is all fine. I have thousands of miles, and several hundred major rebuilds on dozens of different 50cc engines
As long as you go step by step, and take your time figuring things out... Your bike will last as long as you want it to.
Just as a guess it is possible someone took your CVT parts (Variator) and swapped it for one from another bike. Lets say someone bought a Malossi Variator, and wanted a bigger bike. They swap out from who ever bought the good stuff, and set in the older stuff. Whatever... It really don't matter now.
I'd start w/ a set of rollers/good spring, and an 82-85 main jet. With everything else adjusted right the bike should run 40-45 all day every day.
If you want more go up to a 70-90cc big bore, and get 50-55 all day every day.
I have a GY 6 that is 90cc, and I ride 55mph everywhere/everyday. Going on 4 years now I had one crankshaft fail (left side bearings), one variator fail (due to a broken case on a Dr. Pulley slider), and two carbs. . all together I am still under 150 bucks into the bike for parts. (forgot 3 spark plugs).
The engine is fool proof. Meaning even a fool can't destroy them.
Adjust your valves, install a set of 5.5 rollers (or 7gr Dr. Pulley), and a control spring. Take it from there to see what you need/want from the bike. You should be seeing 40+ mph.
OEM you are lucky to see 30-35 mph