Don't know about (free) download but Myers do a nice one.
https://www.myersengines.com/engines/steam_aircraft_engine.htm
If you are just going to just run on air then the Linley "Halo" might be found on the net, that's a 5cylinder. Could be altered for steam by changing the pipework to something that won't be affected by heat.
The Cygnet is a well proven design. Why not do a double rox, six cylinder version? witha hex crankcase, the design is begging for it! Kirk at Hemingways is a nice chap, and he'll sell you extra castings if you ask... He might want some photos of the finished engine though!