Hi Chris,
I don't know if that pack will fit inside your frame, but you should be able to make a cardboard cutout of the battery outline based on these measurements:
Don't forget that the battery will probably need to slide approx 25mm (1") and also have sufficient room inside the frame to lift away from the mounting plate after sliding.
As an alternative, this
52V 30Ah triangle battery pack looks like it might fit within the frame:
It just about looks possible to squeeze 160 genuine 3000mAh capacity 18650 cells and a
BMS unit within those dimensions:
Hopefully the stated 30Ah capacity is correct, but the stated capacities are not always genuine. Check out
this thread for more information.
Unfortunately, the stock rim supplied with the bike is unlikely to be compatible with the Magic Pie, as it appears to have only 32 spoke holes instead of the 36 holes that would be required to install it on a Magic Pie hub.
Alan