The railroads that owned the locomotives, before the foamers "saved" them, rebuilt engines all the time with new parts. As a classic example, most of the original 1881 Baldwin D&RG 406 "Cumbres" has been replaced from the inside out (boiler in the teens and most of the rest of it after the C&S wreck) and it has pulled a variety of tenders over it's long life. It is still D&RGW 346. So why, just because it is now "preserved" should any part of it become holy?
An operable steam locomotive with new components faithfully duplicated from the old is far preferable to a dead cold stuffed and mounted museum relic. The real railroads rebuilt 'em from the ground up so why shouldn't a museum? Just like grandpa's axe. We've replaced the handle three times and head twice but it's still grandpa's axe.