So having taken the frames apart, cleaned them, put them back together and discovered part of the problem was the frames rubbing on the rolling road, I now have the first part of the motion fully assembled. This involved gluing the cranks onto the axles, threading the bushes onto the crank pins and then the coupling rod. On the rear axle this is currently all held in place by a small bolt (until the rest of the motion is added), while on the front axle the screw has been cut to length and the bush permanently secured in place. There were a few worrying moments where things didn't seem to want to run smoothly, but with a little patience it all seems to be working nicely.
As you can see I decided to paint the cranks before assembly, although some of the paint has clearly rubbed off as I assembled the motion. Touching it up should be easy though once I've assembled the rest of it, and if not a little weathering powder should blend things together nicely.