With mine, I brought the image into Sketchup, drew a rough wheel to the correct size, and then just re-sized the image so it matched the wheel sizes. That then allowed me to draw out the profile shape at the center of the image, them move down the length of the image, extruding each section, re-sizing the extruded face with each section, tweak the profile and repeat, down the length.
I hope that makes sense, but let me know if you'd like to see the tutorial video I used.

Thanks, that’s kind of what I ended up doing too!

I think my rear axle is a little close to the seat back, short of fitting the chain drive externally… a bit more tweaking then I think I’ll start on the CAD (cardboard aided design) stage

Mac