The engine you have been gifted is a Solar #1 either originally by Solar Engines of Phoenix Arizona or more recently by PM Research of Wellsville New York. Almost always seen in green, red and bare metal, yours appears to either have been repainted or was perhaps originally an unpainted kit that received the treatment it now wears. They are likely the most common model of Stirling Cycle engines to be found, but actually offer a unique and rather amazing history that stems clear back to the 1950's on the Isle of Man in GB!
More info is available should you be interested.