several things to check. first is very simple. check your sparkplug, replace it is probably the best bet.
next, is the carburetor. it has two important jets in it, a main, and pilot jet. the pilot jet basically runs things from idle to about 25% throttle, give or take, and the main is from there on, or a certain engine speed and above.
if either,or both are dirty, you're not going to have a good running bike. remove and blow them out if you can with an air compressor. if you cant see light through them, they arent clear.
also clean the bowl of the carburetor as well. any dirt or sediment in there, will get stirred up while the bike runs, which will result in poor performance and rough running/lack of power.
also, have the valve clearnces checked. its a pretty simple job, but must be done with the motor cold, like dont even start it in the morning kind of cold. a valve out of spec will make a bike lose power if it isnt sealing entirely. easy adjustment.