According to the FRA, a steam locomotive has to be torn down completely, ultra sounded, and a thorough inspection after 1472 days of service, or 15 years, whichever occurs first. So at the very least it would have to be torn down, boiler tubes removed, and new ones put in.
As a former mechanic of both steam and diesel locomotives the 611 is way over the hump for several reasons...
All the parts are accounted for and were in formally running condition.
It was in good running condition when it was donated to the museum.
It was current with the FRA the last time it was steamed up.
The paint is in good condition and the side panels are not rusting away.
The air brake system is in working condition.
A big advantage of the 611 is that it has roller bearings both on the side rods and on the tender. Roller bearings rarely seize up and require minimal maintenance if any. If the 611 still had friction bearings it wouldn't even be allowed on the main line as friction bearing axles were outlawed decades ago.
One big negative is that the 611 is a coal burner. This would probably be one of the main problems of returning the 611 back to steam.
If you want my 5 cents on the 611 as a mechanic, I would say the tender as it stands is pretty much ready to go. It has roller bearings, and the braking system is working. Flange wear would have to be checked with a gauge and the air brake system would have to be checked for leaks. It would have to be filled with water and see if anything leaks.
Running gear appears to be in good condition and would probably need minimal work done.
The piston rings would have to be replaced as there is no steam lubrication when it is moving back and forth in the yard and the rings have probably worn down.
Boiler tubes cut out and new ones put inside.
Frame was probably already magna fluxed and FRA approved. I doubt if it would need any work.
Gauges would have to be re-calibrated and tested. The piping is probably still good.
Estimated time with 10 people working 40 hours a week. 6 months.
Estimated cost labor $192,000
Estimated cost parts $160,000