IMHO...
Every 1472 calendar days an operator is required to strip the locomotive, removed the lagging on the outside and the flue tubes on the inside, needle scale the interior of the pressure vessel, mark a grid on on pressure vessel, and then take an ultrasound reading at each grid point, which measures the relative thickness of the metal. Calculations are then done to determine if the pressure vessel can be safely operated at its rated pressure. Obvious defects, such as heavy scaling, warping of sheets, cracks, too many broken or eroded stays, pitting of the metal, etc., etc., can disqualify a boiler as well. Most defects can be repaired (if you have the money), but overall thinning of the pressure vessel means it is new boiler time. If the boiler passes its 1472-day inspection, then it is good to go for another 1472 days. This is a gross simplification of the process, and needless to say, it's pretty d--n expensive.
One of the factors in the potential restoration of any of the 490s is that there were dramatic improvements in basic metallurgy over the first three decades or so of the 20th century. The boilers on the 490's are what railroaders used to refer to as "old iron."
Kelly Anderson might want to chime in here.
Mike