Ask for a fair buyout in the form of an IOU. They can pay it when they have more cash or default when they fold. You've locked them in to sticking with the work

Ask for a fair buyout in the form of an IOU. They can pay it when they have more cash or default when they fold. You've locked them in to sticking with the work that you have done by dint of the SEO juice. If they want to reap the benefits of that, they have to compensate you commensurate to what was agreed.