Aren't DC brushless motors just normal 3 phase ac motors driven by half a variable speed drive?
If that is the case then the weight of a 100hp motor is going to be something else. We have a fair few at work and they come in at significant fractions of a tonne. Yes you could shed some weight from the casing but the copper windings are going to be a serious weight. Unless you intend to run at 400vdc then the current draw for full power will mean some pretty heavy gauge wire involved.
Also aren't LiPos prone to failure or worse unless charged very carefully? So not only do you have to lug a shed load of copper into the air you then need to drag along a complex battery charger as well.
I suspect that this sort of thing won't be certified anytime soon the market is far to small to recoup the cost. It may take off in a small way for permit types or more likely EAA experimentals but spam cans won't be electric without some major advancements in superconductors.