I had the same tire problem with my 20 year old original dr. mower . After lots of hours online ,I found a solution,and now my mower has brand nwe tires that look and work great.
I found that the measurements for a wheel sold by Northern tools ,actually a cart wheel was close enough to a match and ordered one wheel as a trial. It only took me about ten minutes to remove the old tire ,remove the new tire from the new wheel, and remount the new tire onto the dr. wheel. .
I have relayed the info to Dr. service . The wheel you want to buy is the 20 in. spoked wheel with semi-pneumatic tire. Item number if you order online is 145121,but instore just ask for the 2o in.spoked wheel.
I have pictures of my mower with the new tires and also some of the mounting procedure if you wish to see them. It is really easy.
Happy mowing
aubreyb1929@windstream.net