318 Magnum Dropped Valve

wow, man that sucks.

the chances of damage beyond the top of piston are small, but they are also not zero.

however, the chances of you needing a valve guide are probably close to 100%

if it were me, i'd clean the top of the piston with burr and polish it nice so there's no hot spot, have the head checked out and replace what's necessary, knock it together and ship it with fingers (and toes) crossed.