As I understand it, Drumboe's boiler barrel is fine, but smokebox, tubes, tubeplate, inner firebox, stays, foundation ring, etc. will all be wanting replacement. That's after you deal with the bottom half.
I reckon it'll be about three million Euro to get it going, allowing for inflation.
DRC has always been hampered by not having the land to relay a section of the original line, something they say they will now attempt to address. That will consume many more Euro.