Giovanni 49cc dirt pocketbike

Hey guys,
Well about 2 weeks ago I bought my first pocket bike (Giovanni dirt pocket bike 49cc) , I rode it slowly for the first 2-3 tanks, but even now, after about 6-7 tanks it goes slowly (starts vary slowly, and top speed about 35km/h).
I'm about 55kg (120lbs) but it fills like its to heavy for it
so my questions are :
1. How can I boost its power ?
2. What parts do I need ? (and where can I buy them)
3. There is some black liquid leaking from the exaust , I thought its becouse there is too much oil in the gas ( first 2-3 tanks > 1:25), but now I'm mixing about 1:40 and it still leaks . Is there something that I can do about it ?

Thanks in advance for all your replys .