After watching the video of the expert from SACA on the subject of the steam engine, converting a 1 piston 1 Kw 2 cycle engine genset might be a really easy way to go here. A metal insert under the cylinder head to consume deadspace could allow an increase in the compression ratio easily. Replace the spark plug with a valve and then seal the shaft to the generator part reasonably well and that's the bulk of it. It already has the exhaust port built in.

They are very cheap, I've seen them on sale for $99, and as stipulated they can be purchased at a hardware store.