Hi Pippington and
to the forum.
GM supply both 140mm and 160mm brake discs but I don't know if the hole spacing on the Magic Pie 4 is the same as the Shimano 6 hole discs.
The original Magic Pies had two sets of mounting holes to cater for two different bolt hole fixing diameters.
If the non-Shimano disc lines up with the bolt holes and is of the correct diameter it should be compatible with your existing caliper. Make sure the fixing bolts are not too long or they will damage the controller if they rub against it.
The front Magic Pie is designed for 100mm spaced dropouts so it should fit without any fork bending, so the services of
Uri Geller should not be needed.
If your front forks are aluminium
(or the dropouts look weak) I strongly recommend fitting some torque arms to cope with the Magic Pie's torque under load, and the opposite turning forces acting upon the axle fixing during regenerative braking.
Alan