You can also set the needle valve to the next notch down to make it a little richer after you replace that spark plug...If all else fails after this, I would be looking at that MZ-65 knockoff pipe your using...Unless your really, really, good at tuning a pipe onto one of these bikes, you might want to consider going back to a stock pipe or pop open the wallet to buy a really premium pipe to get things running better.
Some of the guys here in the forums have done so quite successfully but they are the exception, not the rule...Maybe they will chime in about this pipe your using and how it effects the running of your motor.