Well, it seems I forgot to thank Vince and George for comments. Thanks guys.
I never wanted to take the time to produce a full set of drawings for "Robby",
but I was inspired to learn 3D CAD when I discovered OnShape. I think it was
someone on this forum who suggested it.
(OnShape is a 3D CAD program which operates "in the cloud" and doesn't require
any software on the user side except a modern web browser).
I've now managed to create a 3D model of the engine in 45 parts, not including
fasteners. Unfortunately, that still leaves undone the task of creating 2D
PDF drawings, which would be (I think) the preferred method for distribution
on the forum.
The creation of undimensioned 2D drawings in DXF would be trivial, but the task
of dimensioning them is largely manual and doing it well and properly requires
time and skills I don't really have.
I have made the 3D model public on the OnShape site, and anyone can access it
there.
Here is a link to the OnShape document. I'm pretty sure you don't need an
OnShape account just to view it.
https://cad.onshape.com/documents/576dae70e4b0cc2e7f468d75/w/b5ce5fc2ab3e5ffbb7388406/e/eaa5728c2260a854c8039794OnShape users are free to make a copy, and do as they wish with it, including
modifying it for their own uses.
If you don't wish to use OnShape, but don't mind signing up, you could sign up
(free), grab a copy of my public document, and export it in a 3D format that's
usable in your own 3D CAD program. It appears supported export formats are
Parasolid, ACIS, STEP, IGES, Solidworks, STL, and Rhino.
Hope this is interesting or helpful to someone,
--Tim