A good place to start to understand what is legal under the code of federal regulations is 49CFR230. [
www.govinfo.gov] ASME added back into the boiler and pressure vessel code a section on steam locomotives. There are several people on this forum that have many years of experience repairing steam locomotive boilers. I hope some of them may add to the replies. A short answer is a small area an be built up by pad welding. A large region either needs replaced or the allowable boiler pressure reduced sufficiently to maintain the required factor of safety of 4.