Re: you're favorite upgrade
I think I have two favorites:
My favorite upgrade for peace of mind is my Sportsman 56t sprocket (I'm on a 4stroke) and adapter clamped onto my hub. It is one of the only parts on the bike that I haven't had to mess with at all since I built it. No fiddling around with it, not one tweak, it just works. Well made, quality part.
My favorite useful part is the SBP twist throttle. It just feels good in your hand. The initial twist throttle that came with my huasheng kit looked cheap and felt cheap, and broke like the third time I ran the bike. The SBP throttle does what it should and does it well. Nice and smooth, no sticking.