I have been unable to find a direct parts list for mag wheels on the B3 or B4.
I have got one for the G model which I believe will fit in the B swinging arm. However the G model is a 16″ wheel with drum brake.
There is a good possibility that the wheel spacers on the left of the B and G axle are the same and that the spacers from the right of the B axle will also fit to complete the conversion.
Please let us know if this works