I would wonder how they could take it back if you already titled it? It's your property at that point they would be seizing if you didn't voluntarily surrender it.
They handle this like imenent domain seizures or firearm "buyback" schemes. You're required to hand it over, and they determine the compensation. In the case of MVs the compensation is typically the final bid price. So yeah, you bought the parts truck for $4250 and then put $60k into restoring it to like new, they're going to give you the $4250.
This is unrelated to EUC. They can retroactively change the demil code on any item.
They have various means of coercion to ensure compliance. Everything from coming and forcibly taking it (highly unlikely), to just revoking your title and placing you on a list of persons unable to do business with the US (highly likely).
As a general rule of thumb, you don't really *own* anything the government knows you have. You possess it at their whim, which can be changed at any moment. This applies equally to vehicles, real estate or anything else that is "registered".