The inspection reports should deal with the mill while it is running and while the mill is stopped or at rest. An old mill that may go back a hundred or several hundred years, may contain little of its original fabric. For safety sake replace as much as necessary with new safer materials. If a mill is restored to a particular period, you should carry out maintenance on the same manor as they would have done in that period. Mills were built to make a product and earn money.
Mills are commercial operations, while museums are educational institutions.
When a mill becomes an museum its product is to product history, and a byproduct of that maybe interpretive demonstrations. It may be hard to justify the expense, without making that profit. The mills were not built to be museums and to have tourists. But if that is how the mill may survive, simply to show how it used to be, you need proper operation, safety for visitors and staff, regular maintenance. A miller and a staff who's not afraid to get their hands dirty.