Turn it over again and peel the neoprene sheet off.
There it is! The shaft broke out of the base, and the motor cracked the mounting plate.
There wasn't enough clearance to weld a plate on top of or below the existing plate, so I opted to cut out the broken part and weld in a fresh piece of metal.
Step 2: Cut out the bad parts
Using TechShop's Flow Waterjet cutter, I first sliced a piece of 1/8" scrap steel using the template I created. Next I positioned the damaged base plate in the exact same position as the cutout piece, using the 4 holes where the motor mounts as references. I cut the bad metal out, and got a perfect fit.
Not shown - marking the rotational orientation of the center part -- the motor must be positioned such that the anchors still fit in the holes near the edge of the rim.
Step 3: Weld in the good, and clean it up
After welding i sandblasted the piece. Since it was previously powder coated, the red finish was stubborn. I decided to leave it rough and try a non-standard powder coating technique.