I wouldnt ride it anymore, until you can rule out a blown head gasket or cracked head. Check the oil... there may be coolant in there as well. The coolant chamber is being pressureized by the combustion gases, and thats why its coming out of the overflow tube. If you have coolant in the oil, it wouldnt take long to ruin the rest of the motor.
If you dont have the money for a leak down test, at least check the oil and check the compression, and check the spark plugs. If one of the plugs, looks steam cleaned, then it probably has been.

 

Wildrides, did you get your knock figured out or repaired?

 

not yet no time