Start with the responsibilities that would be left uncovered, rather than searching for someone to copy the founder. Consider what information, authority and skills those responsibilities require.
Developing an internal successor, bringing in external management, admitting a new shareholder and selling the business are different paths. Each demands preparation and a workable governance structure. Plan for an unexpected absence as well as a gradual departure. The central question is how the business can continue making decisions and serving customers when the founder is no longer available.