I can add that KaCio in China makes a mini steam engine-- in fact they call it the "Mini"-- it is 2 cylinder single-acting wobbler, 6mm stroke x 9mm bore(.5cc), and it runs like a champ from my first experience. I steamed it from a Saito OB-1 boiler as you can see in this video:
https://www.youtube.com/watch?v=oyRWCz_Tv04This little 2" long engine is like a gem-- it turns easily, comes complete with a tiny oiler, and is truly an amazing value -- I bought it from Enginediy.com for only $77 USD.
I know there are other 1cc engines made in Europe but I don't know details. (JVC?) and I don't know the Hielscher, sorry.
When you find a mini engine please let us know its size, and the size of the boat you intend for it. Also a Mr. Les Proper on YouTube has several tiny engines and he shows you how to make them, and the boats they can go in.
-Carl