how can i fix my piaggio NRG xtreme moped?

over the last 3 days my moped has decreased in power significantly to the extent that now it will bearly move. you have to drive it around for a few minutes untill the enguine heats up enough, then the ped will start driving properly, or at least reasonably. i have checked the oil, water, drive belt, bearings, spark plug, exhaust (for blockages) and throttle cable. i cannot think of anything else which i can look at (within my mechanic capabiliy), can someone please help!!!!