Step 1 is to run 3 or 4 tanks of fuel through it to get the rings seated. Lots of people neglect that step.
Now you need to decide where your desire for speed and your mechanical ability meet.
If it were my engine, I'd:
-Tear it completely apart
-Balance the rotating assembly
-Gasket match the transfers
-Reshape the intake and exhaust ports
-Clip the bottom of the piston at the intake to where it opens at 60 degrees after bdc.
-Expansion chamber exhaust (modified KTM 50, not some banana pipe POS)
-Fred head (CR machine)
-19mm walbro diaphragm or 19mm Mikuni (PZ19 is the cheaper clone) carburetor
Since it's a 66cc engine, it will have the torque. Change the rear sprocket to a 36t. You'll have to pedal a little faster before engaging the clutch, but I believe in you