Re: Going to buy a bike kit
To learn, the best is probably a kit. You'll also quickly learn that almost everything in the kit is suggested to be replaced. NGK spark plug, better cdi wire and cap, new throttle+kill switch, better sprocket mounting method (depending on your situation)... But most of all, get rid of the included tensioner as fast as possible. Or modify it. More often than not, the tensioner will slide into the rear wheel spokes because it can't clamp properly. These engines can pose a lot of problems, but from a learning perspective you'll learn a lot from it. Some people here suggest taking the top end apart (head and the jug) and making sure that there is no junk ontop of the cylinder. I personally did this and found gasket material on top of my piston. It was also a great opportunity to re torque the head bolts.
And finally.. I would say blue thread locker is a must. You'll have bolts rattlin loose all the time without it and then potentially breaking studs. Of course there is a certain torque range where you may not need the threadlocker but I personally do not know what numbers those would be.
If you're confident in what you can do in terms of fabrication, you may feel up for getting a different motor. Mounting the motor is the hardest part most of the time for any engine that is not a "china girl" as it is called in these kits.
Certainly is fun. Now that I have put my kit together and have found that the steps I took have created a reliable bike, I plan on stepping it up this summer with a new build using a real engine with 9hp. Either dax's or a morini.