top end rebuild can rev well?

i just changed my piston cylinder and rings for a new set (50cc) its done 40miles but i cant get it to rev over 6000 unless its on the stand and it will enventualy break past 6000 on to just under 10000 the bike would do 45 on flat but now will only do 30mph and i dont seam to have a powerband anymore . is this just what happens with a new top end or is something wrong thanks

 

You're supposed to run the bike in for about 75 miles, going really gentle on it. You could have damaged the new top end by going so hard on it, but you might be lucky.