Presumably the website identifies the company that "owns it" somewhere on the "about" page. If you can prove the registered company has gone bust, that may be sufficient.
Also bear in mind there are (presumably) other clients in the same position, probably with the same registrar so the ground work may already have been laid.
Irrespective of anything else, you should email the registrar, explain the situation clearly and concisely, and ask what their procedures are to rectify the situation. I imagine most registrars will have encountered this problem before - you may be imagining problems that aren't there.