This is a freelance 00 tender engine body that fits on the Hornby 0-4-0 "Holden 101" chassis. The Caledonian Pug (Smokey Joe) chassis may also be used (it is identical) but the Holden 101 is recommended because it includes a weight that fits in the smokebox. These engines are readily available (in good condition) second-hand. Do not use Hornby's inside-cylinder 0-4-0 chassis.
This is a freelance design, but its shape and size are not unrealistic. The Victorians made a variety of small tender engines (https://commons.wikimedia.org/wiki/File:Furness_Railway_No_20.jpg
for example) but makers of ready-to-run locomotives tend to concentrate on the bigger 20th century prototypes.
In addition to this locomotive body, you will need a tender. A matching 4-wheel tender (https://www.shapeways.com/product/79FGLLQJU
) is available for this purpose and must be purchased separately.
To remove the original locomotive body from the chassis, work plastic strips or wedges between the cylinders and the valve chests. It's better not to use screwdrivers, which can damage the plastic. Guitar picks work well, if you happen to have some.
To put the new body on the chassis, position the back end first. The chassis has two hooks behind the motor, and these attach inside the firebox. Then push the front end down until it clicks into place, taking care to keep the connecting rods inside the cylinders and making sure that the curved "wrappers" go round the cylinders.
I have successfully mounted the body on three different chassis of varying ages. You might find that the loops on the end of the motor retention springs get in the way, in which case you could trim them with wire cutters or bend them out of the way. Apart from this, there is no need to modify the chassis, but you should remove the hook from the rear coupling. There will be no difficulty in removing the new body and putting back the old one.
The model can be spray painted, or brushed with several coats of thinned hobby acrylics. Gloss acrylics are more durable than matt acrylics. After painting the glossy finish can be flattened with a coat or two of semi-gloss varnish.
The cab roof should be glued into place after painting the interior.
This post on the RMweb forum has more details about painting and lining: