it is a well known case on dz's. Caused by either violent backfire or use of a very powerfull starter with too much fuel in the cylinder. Look at the carbon- builds around the edges....It is a relative old mishap and the motor can run for some time before the power fails. The piston, by the way, shows signs of excessive wear on the lower parts indicating that piston as well as liner and ring needs replacemant.
And I'm not with YS :-)