I too have the Kontax Nano based on the Mason - Guise Stirling cycle, and I'll simply have to agree with everything said ... they are a fantastic little engine with truly impressive operating characteristics.
Recently acquired a low temperature Stirling kit by Inpro Solar, which appears to be a German company that may no longer be in business. I received the kit as a sort of freeby add on to a Solar Engines of Phoenix Arizona group purchase that I managed to work a deal on recently, but I haven't built the Inpro yet .... maybe today?!?!