I can tell you about one of my motors, it's in my gallery as "my second engine", its based on James G. Rizzo's Dyna, from his book "The Stirling Engine Manual". With gas/LPG it has developed 5 Watts, and using a large freznel lens (made for magnifying a TV set), about 5.5 Watts, this BETA motor has a bore of 1 3/4", a power piston stroke of 3/4", and a displacer stroke of 1 3/16", best speed for power 600 to 700 rpm, and it has exceded 2000 rpm, it has done well over 2000 hours. It will produce about 2.5 to 3 Watts at 12Volts on the generator, enough to power a cooling fan on the radiator, and a transistor radio, or a power hacksaw, the one that is fitted to my Ross Yoke engine in the gallery.
An estimate of power can be about 1 watt per cc (I think, or was it per cu inch), what ever, it's probably optimistic. Ian S C