Further along

With the slot in the yoke opened, threaded holes made in the crank disc, the follower 'block' and in the base for the standards, assembly starts to make it look decidedly engine-like and, thankfully, all is reasonably 'square'.

Decision time for the valves now and I think the piston variety will cut the mustard and be easier to make. Aside from the usual annoyance of custom made fasteners there's only the valve block, valve and eccentric gubbins to do.