The adapter plate design is progressing very slowly. I got a quote for the required aluminium. While the raw material might cost about $80 if you buy it by the tonne, a little piece like what I need takes 3 days to cut and costs $400. Offcuts are much cheaper (but you have to go down and root though their bin), which is more than enough encouragement to persevere with the design. You can see my progress at Adapter Plate
I've had quite a lot of trouble getting an accurate outline of the bell housing. Scanning with my A4 flat bed scanner wasn't very successful -- the 4 separate scans didn't stitch cleanly. I've ended up scanning the squiggly bit at the top of the bell housing in one go and adding a circle of the correct diameter. This has worked quite well.
Next up is tweaking the shape of the outer diameter. Once I'm happy, I'll have it water jet cut in MDF to confirm.