This video shows/explains one way to convert a small 4 stroke engine to run on compressed air, in this case at least. If you get an engine to run on compressed air, it will also run on steam.
The key is valve timing. A 4 stroke won't work right out of the box because the intake stroke becomes the power stroke, and what was the compression stroke becomes a jam since it tries to compress the steam. What is done in this video is the camshaft is modified to turn it into a 2 stroke engine. I will let the video explain the rest and am now giddy to try this on my dads defunct 5.5hp tiller engine.