Bonjour Mark,
As a French steam passionate who buys for his collection, I have several Anton
http://www.anton-vapeur.fr/ and several JMC
http://jmc.vapeur.free.fr/ steam plants. Both of them are reliable and just for me, as you talk about the Cristal :
https://www.youtube.com/watch?v=42f1uoF22NU I would suggest you also to look at the JMC 3 cm3 :
https://www.youtube.com/watch?v=JBwO-jLAfnI which is in my Lechalas :
https://www.youtube.com/watch?v=46kYI-yJyB0The two engines also exist in 4,5 cm3 for Anton and 5 cm3 for JMC (in my 1,32 m / 11 kilos Jacques tug ).
When I started in my passion, I had in mind 1 kilos = 1 cm3, and with experience, at our scale, it can be wrong, our steam plants are too powerful and our boats too fast. For example, I have a 16 kilos Fulgerul which is equipped with two 2 cm3 Quartz Anton :
https://www.youtube.com/watch?v=3jombigGCM8.
To my opinion, the impottant thing is to find the appropriate propeller and boiler to the engine, large enough, but not too much, to let the engine run gently and save steam for autonomy.
Besides this, a tug needs power and, if you build this 1,20 m / 27 kilos Anteo tug :
https://www.youtube.com/watch?v=D1B6etQ30b8 I fear 3 cm3 will not be enugh.
My 21 kilos / 1,20m tug JAN has a Stuart D10, but I navigate pulled by it at 1 bar !
https://www.youtube.com/watch?v=7l_-js-fX_I There is an alternative quite less expensive in China but it is a 6,78 cm3 :
http://www.microcosm-engine.com/ and the boiler sold with it is too small for me :
https://www.youtube.com/watch?v=TO16806-3pMIn France, I confirm, we do not go to an official certification organism for our little boilers, it is not a problem, they are 1,5 mm copper thick and can run without problem at 10 bars (Anton and JMC test them at this pressure).