Hole in piston

[Charles Levien]

Hi all, have a friend new to Austin sevens, he had just got his newly acquired 35 tourer on the road and joined us today on our annual Rally. 20 miles into the run from home, engine noise and some smoke from engine bay, photo below tells the story. Question is, what may have caused it? and is it viable to replace the piston, hone, and new rings all round. 
Any suggestions greatly appreciated.

Off the top of my head - running with weak mixture / over-advanced?

Replace piston - certainly. Just check nothing else was damaged (where did the bits of piston go?)

This happened to me with my first Seven ( a little while ago) only in my case enough mixture got into the crankcase to explode, sending the oil filler cap past my head at some speed. I heard it tinkle halfway down the street but never found it.

Charles