The engine looks like the HuaSheng 142F, which is actually a pretty good little engine. However, the final drive ratio is usually the main reason for lack of power. In some cases (and looking at the size of the rear sprocket it may be the case with yours) it is geared too high for the engine to develop peak power. I have the same engine on one of my bikes, but coupled with a different drive and a large (64-tooth) rear sprocket. I'm a heavy weight rider and I get very good performance with mine. Also, that stock exhaust robs power. I replaced mine with a flex pipe exhaust and it increased performance, but made it much louder.
So don't write off that little engine, but I'd suggest looking into a different drive.