If your current business is closing, you can change the adopting employer to a new business to continue the Solo 401k uninterrupted. This avoids any taxable distribution of funds or assets.
The Solo 401k is the most powerful retirement vehicle out there. If you can create a new business that allows you to keep the Solo 401k – then do it! Qualifying business activity today can be anything from a full-time business to very part-time work as a sole proprietor or freelancer.