You might want to have a look at some of Elmer’s Engines. The plans are available free on the internet. Some of the parts can be kind of small, but then so are the materials required. I have completed 4 of his engines. Great way to learn machining methods and so far all have run well.
A 3D drafting program like Rhino 3D can be very helpful in working out the geometry of all the moving parts. Bit of an expense and it requires learning how to use the software.