I hate to play the devil's advocate here, and you do need to jump on this before these people file for bankruptcy, BUT
in many places this is not legally fraud or theft until it can be proven, and bankruptcy will not be, either
No legal entity is going to let you, or escort you somewhere to get "stuff" that you say is yours. So unless you have some pretty good proof that your engine is "your engine," it's not gonna leave.
A pair of heads on a bench might be tough, unless you have previously marked them!!!
Crank, other misc engine parts will be impossible to prove.