This is a model of the Foden tractor "Mighty Atom".
The boiler and engine are from the Wilesco traction engine. The boiler is approx 165mm long x 45mm diameter. It's not easy to measure because it's enclosed in the bodywork. At the rear end is a typical Wilesco round water level sight glass; obscured by the rear platform which has to be removed to view the water level. Also fitted are safety valve and throttle but no whistle, all pretty standard Wilesco style. The safety valve is in a somewhat inaccessible position and is difficult to unscrew if it's tight or to tighten by fingers. I have had made a similar valve but with a hex heed instead of the normal knurling, which can be (un)tightened with a box spanner.
The steam pipe comes out of the top of the throttle, round a loop to the under side of the valve chest. The valve is the usual Wilesco type, sometimes called a "Piston" valve because it's round but is not a true piston valve, closer to a slide valve but round in shape. A drip feed oiler is fitted on the top of the chest. The valve controls steam into a double acting cylinder, and is itself controlled by a slip eccentric on the crank shaft.
The piston is 13mm diameter (assuming it's the same as the D16, which is a good bet). Stroke is measurable at 16mm. The main piston rod is connected to the crank via a con rod with no cross head. The crank and slip eccentric are together at one end of the crank shaft. At the other end is a 55mm diameter mazac flywheel. Exhaust steam is fed from the valve chest into the smoke box to be exhausted through the chimney. Drive from the crank shaft to one rear wheel is via a two stage drive band speed reduction. The front wheels are steered by chains wound round a spiral shaft which is turned by a contrate gear and a spur gear attached to the steering wheel.
The boiler and engine are covered by a half length canopy. Unusually, the canopy is held in place by two magnets. There is a short wooden flat bed platform over the rear wheels.