Hello James,
So many engines, so little time, but here is my approach. First, many model engine designs, even complex ones, can be made from bar stock by a clever chap with a little skill and creativity. It just takes more time, which you say isn't a major factor. Replicating some cast parts can be a real challenge but a challenge might be one of your project objectives.
What I build is to a great extent determined by my building objectives. If I want something simple and quick then a wobbler (oscillator) is a good choice. Oscillators also are a prime choice for work needs, specifically power for model boats. But if I want a substantial project which will present me with a challenge and display (or test) my skills then somethng like a twin horiz mill engine, a twin vertical marine engine, or a beam engine is a good choice.
I'll make a couple of suggestions, the 1st photo below is of the Stuart Models "Beam" engine. The flywheel is 7" in diameter and the cylinder is 1" bore x 2" stroke so it's not too big but not too small. The Stuart Beam is frequently a "second" engine, that is, after making a smaller engine to learn the basic machining processes required to build a model engine, modelers often choose this as a next project. I think this is one of the least complex of the currently available model beam engines but it still requires quite a bit of skill and can be improved (embellished?) in detail to create an interesting engine. Drawings alone are available and a book "Building the Stuart Beam Engine" (A. Smith, TEE Publications, ISBN 0905100662) The horizontal mill engine equivalent to the Stuart Beam engine is the Stuart "Victoria" engine (Photo #3, 7" flyweheel, 1" bore x 2" stroke). All the above applies, including the book availability, and the Victoria also is often built as a twin.
The second beam engine suggestion is the "Sanderson" beam engine. This is an older British design, a model of an engine that actually existed, and is a larger model than the Stuart with a 9" flywheel and 1.5" bore x 2.5" stroke. As a future project of my own a few years ago, because castings were becomming expensive, I went over the design and deteremined that it could be built from bar stock. It won't be easy, but it can be done and will probably result in a better looking engine than if it was made from the castings, which can vary in quality and machinability. This engine has a lot of detail and can provide a builder with a great showcase for their skills. Drawings are available and a full construction article (including drawings) was published a few years ago in the Model Engineer Magazine. Back issues or article reprints should not be hard to find. I know of a couple of "doubled" models of this engine which are very impressive. The model Sanderson pictured below was recently built (by someone else) entirely from bar stock confirming that it can be done.
These are but two of many project designs available. If you want to see a large variety of very interesting model steam engine projects in one place visit the Polly Engineering website and download their catalogue (pdf) and have a look. Naturally their business is selling castings kits but also lots of those engines could be built from raw stock. Disclaimer: I have no interest in Polly Engineering other than as friend-of-the-firm and a satisfied customer.
