Top of the drawing says "boiler copper – all joints silver soldered" Which would be the prefered material
Having said that if its just for use at home then the engine is no larger than a Mamod which have brass boilers you should be OK for occasional use.
I would also strongly suggest you fit a safety valve into the filling bush and not just a plug, again Mamod ones can be bought on e-bay and thread the bush to suit
J
Edited By JasonB on 26/10/2014 18:02:52