First, join AOPA if you haven't already. Sign up for their legal services @ $30/yr + member dues.
I think the FAA issuing a registration has nothing to do with airworthiness. This issue should be pursued vigilantly to reissue a certificate.
Who issued a lien and for what purpose? Did you have a title search done before buying the aircraft? Was it clear? Did you perform all due dilligence prior to purchase?
If so, I don't think anyone has the right to issue a lien to a new owner. If they have a lien to issue, it should be to other property owned by the previous owner unless, perhaps, it was filed and approved prior to sale.
Find the seller and confront him with these issues.
Strange, but you are right. The registration info says the airworthiness is "unknown". Bummer.
