FIns
 

Hey

It looks like it could be. I would clean up the engine thoroughly and keep an eye on it to see if it reappears and if it does how long it takes to reappear. Then go from there. Maybe it just came from the tire kicking up something onto the engine. Grease or oil?

Not super common, but not unusual. Start by checking the head bolt torque. I use the most current CG125 head torque specs. 24ft-lbs for the 4 main nuts under the valve cover and 20ftlbs for the outer bolt on the left side of the head.

Clean it up as BBOWL10 suggests afterward and see if the oil returns. If so, a new head gasket isn't very expensive and very easy to do.

Looks just like a Harley head gasket! Haha

If you pull the head make sure to check if the o-ring for the oil passageway isn't split, smashed or missing.

also make sure the breather is clear, pressure building in the crankcase will get past many seals.

When I was a kid, all my four strokes (Honda's) did this. I just kept on riding and Gunk'd them once in a while.

If that o-ring cracks or changes shape in any way the oil from the back-right cylinder stud will completely strip the head gasket of any material. If the cylinder stud bolts aren't equally torqued at the pounds listed in the CG 250 repair manual you'll also get this..

Get gasket kits off Google shopping or AliExpress. RPS and all the dealers who are basically drop-shippers for them want a fifth what they sale a new engine for a single head gasket..