[Steve Barcak]

I'm very sorry to read of the loss of your motor. It having only 2,000 and no gauges means-

1. something was not put together correctly
2. things were not monitored correctly

I don't want to rag on you so do not take it the wrong way. I'm posting to help, not hurt.
This is something you need to learn from.

Valves do not normally come apart. Cast pistons will disintegrate and often take the motor with it when something hits it or during detonation. If you had forged pistons, the damage would have been minor.
Cast pistons act like glass. They are durable to a point,then, when pushed beyond their limits will shatter like glass as your did. A forged piston can absorb punishment. If your valve came apart, the forged piston would usually absorb it with damage but, would stay together.
Were these used cast pistons? If so, you were taking a big chance.

ALWAYS re torque head gaskets. The center bolts especially come loose over a short time. A few thousand miles was a good indicator.

Learn from your mistakes and do a smarter, better engine build. This has happened to me and , I am sure many others. It's a bummer to say the least but, you have to learn from it and move on.
Very sorry to see what happened to you.