Try to work out the KV of the motor, the rpm/volt.
Then see what you can do with making a circular plate to go over existing cover plate, and use that as your spoke flange. Perhaps there is an existing round plate available, with somewhere in the ball park outside diameter true-roundness, then you just have to deal with the inner diameter cutout.
I think it could all be done in ones garage. Get two pieces of steel and make a sufficient diameter compass, the V thing where you add a pencil to make circles. Be sure to bolt it down tight so it don't move. Get a grinder and have fun! I would leave the middle, so you can use a bolt in the center, and a drill to spin it. Once its balanced, work on the middle part, using the same methods.
Or just buy a kit from YESCOMUSA.com