Check your motor mounts! I was loosing top end power also. The bike, (HT 48cc), would build up good speed until the higher rpm's and die, then picking back up in the middle range. I noticed a daily decrease of a mile or two. I gradually went from 30+ to 22mph, even as I was investigating the following remedies;
I tightened the intake system, cleaned the carb, changed jets, adjusted the c-clip, adjusted the prongs in the carb bowl, installed a new fuel filter, ran a little SeaFoam, vented the tank, installed a new plug, and flushed the exhaust. Nothing! Then I noticed that the motor had a slight shake to it while I was removing the plug again. The engine hardware was not tight anymore so I carefully snugged the motor mounts. The only other thing that I did was spray a little wd40 behind the magneto cover, cdi wire input, (very little), and inside the spark plug wire cap. I did this to drive out any possible moisture that may have entered and compromised a good connection. I then took it for a test ride. She hit a nice top speed again right away! I was just about to give up. (Not really). I am leaning toward loose motor mounts as the cause of the loss of WOT power and strange engine behaviour. Just my 2cents. I hope this helps someone out there having this type of trouble. Good luck.
At least now I have a spare carb, magneto, cdi and plug cap on the way, which I ordered before I found the fix.