Brian, that is a beautiful beast. Some of us are still taking baby steps though…
Marked out a piece of 3mm brass and after drilling and rough cutting finished by milling the ends and scalloped sides, just 'by eye' to the marks. A piece of brass rod was drilled at both ends, but leaving a 'wall' in the middle, and was silver-soldered into a small recess while a bronxe bush was soldered into the other end to carry the crankshaft:

The opposite side was rubbed on the finest sandpaper I had, on a piece of glass, with some fine (ex rc-car, for bearings) oil, to make a decent-ish port face. The steam-ports still need drilling:

I also made a crankshaft. After a bit of fiddling, the crank, transmission, and wheels all turn smoothly. A flywheel is needed on the other end of the crankshaft, and the centre axle needs some work, but this brings the Cracker on quite a bit.

I should have used countersunk screws to mount the standard/portblock, as with this set-up, with the crank-disc standing quite proud with the cheese-heads behind, the cylinder and piston will overhang a bit more than the plan shows. This may be no bad thing though, as it will mean the trunnion will have a bit more meat to grab in the cylinder, on the plan it's a very small fit. But it could end up looking quite clumsy.
Starting to think about what I might build next. Which may be getting ahead of myself, but at least I know that I will want to build something next…