I recommend Stewart Hart's PottyMill as a starter project. It's been discussed many times on the forum, for example here, and the plans are widely available on the internet. I built mine from this version, which is rather cramped and unconventional in glorious technicolor, but I found no mistakes. Stewart is a forum member and a PM to him (see Inbox in Green Banner above), might get you a copy of his conventionally drawn originals. Stewart's version is a good example of a typical model engineering plan. It's worth studying both!
For Stirling and various other engines, try Jan Ridders. (His website starts in Dutch, so press the 'English' button top left.)
Dave