Adverse possession wouldn't wipe out a properly recorded lein. Of course, it doesn't seem that the lein was properly recorded. You could maybe sue the leinholder to provide proper documentation or abandon the lein, but you run the risk of the leinholder finding enough documentation to convince a judge, and making the lein valid and enforceable, instead of vaguely maybe not an issue.

Or, maybe you just pay someone to "find" the documents, and hope the leinholder doesn't notice or doesn't care. This case seems murky enough that even a potential recovery of $1M might not be worth it for the leinholder.