I had to look up the AC-50 motor and controller:
So for $4300 you get a 96 volt AC motor rated 67 peak HP, 650A, 110 lb-ft torque, 6500 RPM, 89% efficiency, and 110 lb. Plus you get a Curtis controller, display, and wiring harness. Peaks at 3000 RPM and 550A on a 90V DC battery bus.
So that's 50 kW or 66 HP. But there's probably a lot of sag on a 90V battery pack. You will probably want about 200 amps maximum continuous or 18 kW or 24 HP. Probably about right for normal driving. I don't know anything about the controller but I assume it's pretty much like a standard VFD except made for lower voltage and higher current.
IIRC I found a suitable 30-40 HP motor for about $900, and it should be able to provide about 2.5 times that for short term use so it may outperform the AC-50. It's probably twice as heavy but still a small part of vehicle weight and battery pack. And a standard 240 VAC VFD rated 30 HP can be obtained for, say, $500. So there's $1400 vs $4300. 1/3 the cost, or, now you have almost $2000 for more batteries. Enough, probably, to go from 90 volts to 300 volts. And now your DC wiring for 40 HP will be just 100 amps, and the three phase motor wires will be about 50 amps, so something like #6 or #8 will be fine.
But maybe the AC-50 is attractive because it is a "drop in" kit. However, it still leaves the battery pack, and a 90V pack at 350 amps for 40 HP will need a lot of really heavy connections. So I'm just trying to see how the cost analysis works out. And I guess I'm still thinking like a DIYer where part of the joy is the challenge of working things out and trying some alternatives.