That is a hard question without some details or pictures. What letters and numbers does it have down by the oil drain plug on the left side of the engine?
Looks like a 2001 or 2002 WC-1 engine. 138cc flat head. This engine is a problem child! The valve seats come loose from the cylinder. This can only be repaired by changing the cylinder to an NE5 or an aftermarket cast iron unit.
Ok Good news!
That is an NE5 engine. The good one! It has a 22mm carb, the aluminum carb spacer, the large crankcase breather hose. Congratulations you have all the latest upgrades that everybody else had to add back in 2004.