Most confiscations I've read about is the 25 year old vehicle rule. That's the one they nab most people on, importing a non us vehicle newer than 25 years.

The 21.year clause is kind of a red herring if you read it because you have to pay for DOT safety certification and nobody is doing that.

If it's just an engine, especially one that is essentially US spec, the rule under appendix section 3.2 of kapcreation's link states you can import it as an auto part. No biggie.