Cat 40 scraper tires

I was looking at my 40 scraper today and it is nearing time for new tires.  My machine has 12.00x21 diamond treads on it and I need 2 more to make it whole in the near future.  Last I knew, a unicorn was easier to find than 21 inch diamond tread rubber.  Has anyone come up with any new solutions to retain the 21 inch rims and pop a tire on them that works?  I was just curious if I had missed something slick that somebody more clever than I am had done or found to work around all the work to upgrade the rims. Thanks.

D4e had a set of 4 21" take offs because one did not hold air to his satisfaction. He posted for sale and nobody replied, so he told his tire man to toss them...
He had a machine shop cut the rims and had 20" centers made for his. Only a couple of coins, easier than finding 21". Good luck. JM

20" used to be available everywhere but now they are getting difficult to find. at a reasonable price.
Does anyone know what serial number series of the 40 scraper made the change in tire size? Mine has 20"
There are some used or NOS of military tires around that work for the 20" but none that I know of for the 21" rims.

I can cut and weld the centers into new rims if I had them on hand. I have 2 of my tires tubed and they hold air fine, a third is just fine tubeless, holds pressure for years, the 4th one leaked air out as fast as I put it in yesterday. I removed it from the rim and am putting a tube in today for the short term. I suppose I will have to find a set of 20" rims at some point. The 21's look better on them in my opinion, but if you cant get rubber, you can't get it.