TR7 engine

["Byron N. Brashears" <brashears@hughes.net>]

I was racing my 76 TR7 Coupe and the engine stalled when I restarted it
there was cloud of smoke and steam. I removed the head this weekend and
found that the #4 exhaust valve seat had come apart and fell into the
combustion chamber. There some 1/8 indentation in the #4 piston. Will
this piston have to replaced or all the pistons replaced as a set? What
are the likely results of leaving that piston in?