Er…… I think the clue is in the title?! 6 1/2" OD by 6.00 ID (I think with my limited maths that's about 1/8" wall thickness) and given its a smokebox probably 4 – 5" long, it's not going to be that long anyway. I would assume the exact material is immaterial but non ferrous probably best for a corrosion point of view. If you got a piece of 1/8" copper plate it shouldn't be too hard to roll it round a former if it's well annealed. Sorry don't have any suggestions for a piece of tube though.
Oops Jason beat me too it! If it's a traction engine then agree with Jason, steel would be best, more possibilities for supply too, I got a piece of 13" OD pipeline tube for mine and turned it down to 12 5/8".
Paul.
Edited By Paul Kemp on 21/04/2019 20:22:01