The pump was a pedestal type, the lower half is only held on with a couple screws. The motor shaft is 1/2 inch with a flat side. I purchased a 1/2 drill chuck threaded 1/2 fine inside (item 345709).
Took a 1/2 fine thread bolt, cut the head off, and ground one side flat. The threaded end of the bolt was then drilled and taped 10-24 for holding the chuck in place. I then used a 1/2 bronze coupling (I know its soft, but it's what I had at the store), drilled and taped each side for a set screw connecting the chuck to the motor. Here's the flex shaft (item 309125)
The mandrel (item 338826)
It is threaded 1/4-20 so I just used a longer bolt with a couple fender washers. If you build your own use a lower rpm motor. I tried it with 6" discs on my grinder once, it just melts the scotchbrite. The sump pump motor is a 1725 rpm with a 3" disc so the face speed is slow enough not to heat up at all. Just for anyone looking for bulk green scotchbrite the item is 629124, 617586 for maroon, and 617595 for grey (its the finest, and green the coarsest)
All of these are on http://www.doitbest.com
or your local Do It Best hardware. And no I don't make a commision if you buy it