Let me clarify- if the cam is free and the engine is still seized then suspect that the rod and / or main bearings are the cause of the seizure.

I have doubts that a scuffed piston(s) has caused the seizure since it usually contracts when cooled down. The engine then is free but since the skirt is collapsed you get major knocking. Extreme piston slap so to speak!

Basically you need to do "exploratory surgery" to see how much damage was done.
A rapid loss of oil under light load may grab the mains limiting the damage however a spun main would likely mean major machine work to align bore plus crank damage.

At this point you will need to decide if a rebuild is worth it or try to find a good used engine.

Good luck
Tom
Senior Service Engineer for Clevite Engine Parts (we make bearings)