So I know the motor is mounted on plywood, and running full steam with a 3D printed part in place of a metal one isn't the best idea... but it works!

The motor spun up full throttle, and I was moving the drive axle. Next I'm going to put it all back together with the wheels and the handles, and see if I can drive this around the yard. I might need to move the motor back an 1/8 of an inch ... but I can do that just by loosening the screws, there's enough play.