Most of these engines are kits that you have to finish off, with either polishing, or some machine work. You can scratch build them from plans obviously. The "package" he is building does not seem to be a kit. The engine may or may not be. Getting a steam engine to work out of the box isn't easy. The car body seems to be scratch built, lots of straight cuts parts, no compound curves. The end result is a really authentic model, and it is actually steam powered! There are lots of ready made components, like wheels gears, and pulleys, flat stock with holes, that you can purchase, and if available, are far cheaper to buy usually than to make, unless you have the equipment. I think that steam launches (boats) make some of the neatest models as they are easily radio controlled and work extremely well. These kits aren't "cheap". The car pictured below costs £207.00, and that does not include freight!
I have always wanted to make a Sterling Engine but they do not scale up or down!
Bottom line is this hobby takes a lot of effort and also gives a lot of pleasure. The level of pre-assembled and finished off parts you purchase are dependent on your abilities and equipment you have to finish off the engine. It all requires skill and it is fun, and the engines could be used on paper models, if done carefully, aluminum foil for insulation as an example.
http://www.mamod.co.uk/shop-categories/1406b-steam-roadster-kit-sa1k-burgundy