You might be geared to high so the thing runs out of torque and bogs down at low rpm,never able to develop max power.Do you know what you overall ratio including any gear reduction inside the engine,it should be.Useful rule of the thumb:
R(eduction)= 800/D(isplacement).For a 45 cc engine ,R =18. If you are interested in speed: S(peed)= .075x rpm/R .For instance at 5000 rpm and R=18, S= 0.075x5000/18 or S=21 mph.On the flats you might get away with a bit lower reduction say 15.Advancing the timing may help in getting more oomph at higher rpm,at the expense of low speed performance.Two stroke engines are sensitive to timing and ignition performance in general and if the points are adjustable it's possible to get it wrong.With late timing the engine will run hot.Does the plug look OK (light tan).