Am I justified in my frustration?

I don't know, however as the details slowly come out I strongly doubt it.
I bought the engine with the cracked heads.
Buy back guy went thru engine, it was just rebuilt. Replaced the heads.
I still have an I'll feeling about using this motor.
Something about having something powering my car that a dirty guy had. Even if he never drove it.
Either way, the “you” in this is scum if he deliberately withheld information and overcharged the original owner with the original owner made to believe he was getting the car back in exact condition as when sold.

turning a profit is one thing, covering up or lying to make said profit is a completely other thing.