how can I fix my Diplomat APL8225 (or APL 8215)?

the water distribution arm, which takes the water to the upper rotor becomes loose at its base when operating, making a loud noise. The reason is that its location to the retaining plate below depends an a retainer collar, into which it fits, and which is broken, so not retaining the lugs on the arm. Can i obtain the plate which retains this distribution arm, to replace the broken one, or can i fix it another way?