Notice how that loader in the photo has fresh black paint on it and nothin' in it? That should tell you all you need to know. Do they have video showing that tractor sitting on its nose with the rear wheels about 1' off the ground? I guarantee you thats what would happen with any load in that bucket.
Putting a loader on a 2wd 20 hp lawnmower is wishing for pure serendipity that ain't gonna happen. The truth is that it will be very slow, barely able to lift itself, and no use whatsoever without tons of rear
ballast. The comparison of your Simplicity mower to a BX with loader is pure rubbish. Save yourself a lot of grief and cost and buy a little trailer you can pull behind it. If you must, make the trailer a dump trailer that hooks to the Simplicity's hydraulics. An add-on 3PH would be infinitely more useful than an FEL on that machine.
BTW: I think you have a great lawnmower that can also have lots of other uses, just NOT an FEL.