I know little about these pumps but I acquired one recently that was seized and after a couple of days of playin' around it's working. I worried too when the rings fell out.
. "Oh great, more stuff for the junk drawer...". They are merely to hold the shaft from spinning, hence the "D" shape on the ends earlier mentioned.
Do the vinegar thing as Jonathon said. Throw a couple of cucumbers in the bucket too while yur' at it...
You may have to remove the impellar housing from the motor unit. See if the motor spins without the housing attached. Mine had four threaded shafts that needed the assitance of "channel lock" pliers to remove. You'll then know which "end to attack..."
When re-assembling, slowly snug the shafts evenly to ensure it doesn't bind-up again. Kinda' like a car tire tightening pattern...
Lube the o-ring too...
