Peter
Good question! A lot depends on what pressure it is run at and how well it's made. The answer is that I don't really know, but I would expect running at about 30psi with paddles 4 to 5" diameter with floats about 1.5" wide would present no problems.
Any type of boiler should be OK - Centre flue: return flue or some sort of vertical would all work well enough obviously provided thy produce enough steam. In order to know how much steam is needed, we need to know the scale speed , the paddle diameter and how fast the engine must run to give that speed. Reckon on about 1/3 slip. Knowing the swept volume of the engine, the volume of steam required can easily be worked out.
I always design my own boilers to suit the particular model. I would suggest "Model Boilers and Boilermaking by K.N.Harris as an excellent book for getting into boiler design.
Is this any use to you?
Malcolm