Bryan, The engine you ask about is made to do work and will do a fine job powering a model steam boat. There are no power ratings on these small engines. Also, power produced is quite subjective. It is a function of bore, stroke, RPM, torque and steam pressure. Steam pressure is the big variable. You can easily find formulas that will help you calculate the output. That said, the horsepower would clearly be fractional. I have one of these and it is very nicely built. I got it from forum member JinMachine. He is the builder and runs the CNC cooperative that makes them.
For comparison, a Stuart 5A with 2.25" X 2" bore/stroke is capable of 1.5 hp with 100-120 psi.
A Stuart Swan which is two 5A cylinders is rated at 3 hp with 100 psi steam pressure.