Dom....as you progress, you will find the following very useful for manufacturing your own self bent tube spools
Du-Bro tube benders for both 1/8" and 5/32" OD tube.....also suitable for 3 mm and 4 mm OD hard metric sized tubes
Manufacturing small tube bends in 3D is an art
....some say for every 1" of correctly bent tube.....[maybe you have used 12" of material]
Refillable gas torch is great for soldering tube connections and annealing tube prior to bending
Are you aware that plumbers soft solder may be suitable for exhaust tubes, however steam lines from the boiler to the engine should be joined preferably via silver solder or high temperature soft solder
There a number of threads here on MBM about soldering
Derek
PS......
1. your engine bed is good that you have a raised lip around the edges, as oscillator's tend to spray a little sloshy water about.....
2. what type of electric driven water pumps are you considering?
3. please remember that the gas tank should be made easily removable [for filling outside of the hull]
4. you may wish to research a little in the actual recommended placement of the gas tank to the boiler may be critical with respect to temperature gradients especially with the cooler ambient temperatures in Europe