Get used to problems, these bikes are high maintenance.
Cold starting...have you tries using 'the choke'? If yes, get your hands on Aerostart or Start Ya Bastard (no joke, thats what its called)
Upgrades? how fat is your wallet? Theres lots of them. If you use the "SEARCH" feature in the top bar of the page, you'll find all the info you need.
I dont know about engine diagrams, the best way to learn is to pull it apart and play with it (The engine, not your wang)
And as for the guide, they're all in Chinglish, mate. The best guide to it is using the "SEARCH" feature as previously discussed.
Have fun with it, they are a blast! If your lucky, you'll get the pocket bike bug like i did and you wont look back, except to see who's chasing you out of the corners
Good luck,
Claw