I bought the all brass horizontal engine 7 or 8 years ago and made a boiler for it, my most recent video of the steam plant is here:
As you can see it runs quite well now on fairly modest pressure, usually 7-12 psi with the safety set to 20 - it is a sizeable boiler but just soft soldered ( I did pressure test it to 60 psi before use!), the governor is just for show, the rest of the bits are largely recycled brass stuff from charity shops.
Dave