Actually, #20's boiler was not that bad. She did well on the 1.25 hydro we gave her except for a couple flues. By the standards and capabilities of the RGS in 1951, she wasn't all that bad from a boiler standpoint. We condemned much of her firebox with the UT machine, which wasn't around in 1951. Fire cracking and staybolt hole stress cracking had been addressed by the RGS in a rather ugly but acceptable manner. We found more cracks but they would not have come through for awhile and the RGS would have just patched them the same way had they continued to operate. The biggest issue for us was the dented boiler which just wasn't a good thing, though it operated for eight years that way.
The machinery was serviceable but pretty tired. I don't think it tracked all that well with the drivers misaligned, but it probably mostly stayed on the rails.
Our goal is to bring #20 up to a very good state of repair so that it can be operated for many years with proper care, in a reliable manner. Personally, I wouldn't have been afraid to fire her up after the hydro and some miscellaneous tinkering. She probably would have run, more or less, for a little while before something broke. That was most likely a good state of repair for the RGS in 1951.
You can probably think of locomotives that were "rebuilt" over the past couple decades and ran for only a short time before they started to develop more problems and often ended up dead in a short while. This is what we want to avoid.