If you have any basic DIY skills a bash valve engine is quite easy to make using a two stroke petrol engine from an old lawn mower or moped.
A bash valve can be made up from stainless steel screwed pipe fittings and fitted through the spark plug hole as shown here.
Also a Stuart Turner Sirrus engine would do the job and these can sometimes be found for sale on ebay. This link shows a bike made by Richard Smith; I think there is also a video on youtube.
All the best for your project
Just a thought; what steam pressures are you hoping to use for the engine the reservoir charge pressure?