Again... Did you lose a valve spring or a retainer as suggested by another responder? Look at the spring for the valve in question very carefully for broken coils. If you lose the spring, the valve will hit the piston and bent the head and eventually break off.
Did you check valve-piston clearance in the mock-up stage during build? If not the valve could hit eh piston and break the head off.
Did you change cam (longer curation or more lift) since the piston-valve clearance was checked.
Note the common thread here is that it is likely something happened to break the valve and the broken valve did the rest of the damage...
Did you use cheap valves or good inconel valves (or at least Manley Severe Duty valves) for your blown application?
Lastly, the block is trashed (maybe sleeveable (?), but I've never done anything but replace a block with a cracked cylinder). The piston and rod are shot (trust me) and the crank may be twisted (have it checked before reusing it). The head is also likely gone since the valve head was definitely smashed against the combustion chamber when it bent the rod and dented the piston.
I had this happen to a very fast kevlar hull, BBC, tunnel jet boat once and saw the valve head spit out of the over-transom headers into the lake. I was several miles out and fired it up and drove it back on seven cylinders. Ran OK, but the engine filled to the top of the valve covers with an
oil/water mix... What a mess...
Sorry to hear of your misfortune. Good luck and do not to forget to be really careful and use the best valves and perform all of the clearance checks and occasionally check the valvesprings if the cam lift is big.