There's one posted here in Plans (or Downloads, I forget) under "Hot Air Engine". The only trouble is there aren't any instructions with it, and it's been converted from imperial to metric along the way, but the basic plan is sound. Marv's mentioned a different design as good for a "first stirling" as well IIRC, though a brief search didn't turn it up. The bonus with the non-LTD sterling is almost any of them will run if you stick a propane torch under the hot end