Here's a page with a bunch of simple stirling engine models that can run on low-grade heat. The one entitled "MC Easy" is built with a "floppy" internal displacer block so there aren't any seals to contend with. The page author's intention is to provide simple designs that can be put to use pumping water or providing small-scale power. Who knows how long this page will be around. Grab the designs while you can.
http://members.aol.com/hstierhof/index.html
And here's another great link with a bunch of "tin can" based stirling engines. The design is pretty robust and can be scaled up as much as one might like.
http://www.boydhouse.com/darryl/stirling/index.html
Enjoy!
