I believe the part number you need is 684481 which are still available through your local Toro dealer. This web site is where I found that number. Please double check your model number as I was not able to find the one you posted.
My lawn boy is a M21EMR - Lawn-Boy Walk-Behind Mower, 1989 (SN: H00000001-H99999999)
and when I looked up the a new wheel they want $121.00 for one. That is why I'm looking for used.
Thanks for your reply.