No single cylinder engine (2 stroke or 4 stroke) can be perfectly balanced because the requirements for perfect balance keep changing as the crank rotates. The purely rotating bits are easy but the reciprocating parts make life difficult. You could weight the crank web for perfect balance of the piston at top and bottom of the stroke but at half stroke the counterweight will be at 90 degrees to the axis of the cylinder. The compromise for minimum vibration is 50% of the weight of the reciprocating bits. The conrod however is peculiar in that basically the big end is purely rotating while the small end is reciprocating. All other points of the rod are various mixtures of the two.

Multi cylinder engines don't have this problem because as one piston is coming down there's another on the way up so their individual reciprocating forces balance each other out.