If you mean a steam engine with approximately a 3.00" X 5.00" Bore/Stroke ....depending on you skill level, your equipment, your imagination, etc....
I would start by searching for companies that sell/make steam engines that will power small steam-launches.
Tiny Power Steam Engines
Pearl Steam Engine
Elliot Bay Steam launch
Reliable Steam Engines
Mike Brown Steam Engines
Many of these companies will sell you complete casting kits, or, just castings for a cylinder, crankshaft, and valve chest... then, make the rest from bar-stock, plate steel, aluminum...whatever. You could also make the whole engine from bar-stock. Buy drawings for a "vertical", and fabricate a new base. Modify the design and make it look like a Mill-type engine. Adapt available designs and make the engine into what YOU want it to be. It can, and has been done, but if you are a just a beginner, I would seek help from those who are experienced with this sort of thing. You will also need a certified, correctly built, boiler to power your engine. Seek help, and heed advice about boilers. Seek help, and heed advice about boilers! An engine failure is one thing....a boiler failure can be deadly.
Try: You-Tube videos for additional ideas.
Try: Scaling up, and modifying some existing designs.
Sorry, can't help you with casting, machining, etc. But if you buy some parts, like the cylinder and valve chest, etc. maybe you will not need to design and then cast some of the other parts. Use Bar-stock, plate, weldments, etc.
Don't be daunted. Go for it, and have fun doing it.