I suggest you email Philip
(sales@goldenmotor.com) and ask if it is possible to purchase the 12" cast rim/motor ring assembly for the 12" Smart Pie on its own:
Aidan's scooter was built using a 16" Magic Pie wheel:
Therefore it should also be possible to use a spoked 16" rim Smart Pie if you cannot get hold of the original 12" cast rim/motor ring assembly.
GM assemble their 20" Smart Pie wheels with all the leading spokes on one side and all the trailing spokes on the other:
Installing the Smart Pie into a 16" rim could be achieved by drilling another 36 holes in the hub flanges to keep the spoke angles within an acceptable range as shown here:
This would allow a more conventional spoke angle with trailing and leading spokes distributed evenly on both sides of the wheel:
If you don't like the idea if drilling your hub, you might be able to get away with radial spoking a 16" rim to a Smart Pie.
The required spoke length will vary considerably according to the spoke pattern and actual rim being used, but you would need to make sure that you can obtain short enough spokes
(or the correct cast 12" rim) before dismantling your existing wheel.
Alan