I Bought one of the mini boilers available on the internet, thought it may be of interest to some.
I modified it by adding an adaptor a globe valve and a home made displacement oiler.
Not sure what pressure it operates at but would guess around 10 tpi
I have a small collection of mini engines and this boiler powers them all although not for long, running time about 4 or 5 minutes
Pictured is a mini muncaster 7/16" bore and going like the clappers
Geoff