Is it possible to repair the "on" button; it no longer stays down without being held.

Replace the reset button or bypass it.

Determine first if it really is the motor. Does it only cut out if loaded or will it happen without any load? Remove all bowls and blades, plug it in and turn the power on and press the on button. With something plastic press the small white button that is in the middle above the switch panel and normally hidden by the bowl. This is the safety cutout switch avtivated by the bowl, lid and pusher being in position. Does the motor run while the button is depressed or does it again cut out after a short time but with button still depressed. If it doesn't cut out it's likely that wear in the complicated mechanisms in the lid and bowl handle are releasing the cutout switch with small vibrations of motor running. I figure out which one to replace by swapping out one of the 2 parts for a new one as I try to carry both parts in stock for that purpose.

Change the power button

Replace switch, sorry didn't mean to say the obvious. Just good like that I guess

It sounds like you need a new working bowl. It is not uncommon for that safety switch mechanism to fail inside the handle.

A gouge in the edge of a plastic button is not the end of the world, it has to come out to effect a repair. I use a sharp knife down the side, you can always blend any little damages to the plastic with the round shaft of a screwdriver.

Well, it sounds like whoever the vendor is has added shipping and possibly tax. Without more information, I couldn't say for sure. I suggest you check the product on amazon and overstock.com to see if there is a better deal.
Regards,
Farmerbear at FixYa.com

I don't know about the little lip you're referring to, but the central spindle that comes up from the base of our KitchenAid 12 cups food processor KFP750WH0 splintered and broke while we were using it. We ordered a replacement motor sleeve shaft part number KA8211849 from the Gourmet Depot for $4.95 plus $3.50 s/h. Removing the broken plastic sleeve shaft was the difficulty; it isn't glued or screwed on but somehow it didn't want to be pulled, pried, or cut off. With the help of our neighbor fix-it man and all four of us working at it with screwdrivers, we got the old broken part up and off. Pushing the new one onto the metal motor shaft was easy, and it was fixed!