Here's another option for tires for conversions - roll your own. If you've got a 3D printer it's simpler, cheaper, and faster than it sounds. I'm including 3 links to YouTube videos.
All of these videos use Oogoo, which is a DIY mixture of 100% silicon caulk, corn starch, and sometimes water. (It has to be the caulk that smells like vinegar as it cures, the newer less odiferous caulk won't work - and no acrylic-blend caulk either.) The water is what causes the silicon caulk to set up. Other agents can be added to thin the caulk mixture, as shown in the third video.
In this first link he makes a solid tire using a 3D printed mold. The clock shows the the elapsed time from start to finish. He's using cooking oil as a release agent, and the ingredients he's using are black caulk, corn starch, and water. This is the link.
https://www.youtube.com/watch?v=E3IRgjFhW6Y
The next video shows a different mold that has a better surface quality, making a foam-filled tire. The release agent was silicone oil. The ingredients in his Oogoo were clear caulk, laser printer toner, and corn starch. It looks like he's making a pretty durable tire. In this video you can see that he weighed the caulk, but I couldn't find any info on the weights of the other ingredients. Here's the link to the 2nd video.
https://www.youtube.com/watch?v=X62rLgvMO5Q
This last video is not about making tires, but rather making the Oogoo more pourable by thinning it with naptha. The one thing that I did like about the 3rd video was that he not ony weighed the caulk and the corn starch, he told us the weights. That way he knew what his pot life and cure times would be. He used 10g of corn starch and 100g of caulk. I haven't been able to find a lot of info on this ratio, it varies from 1:1 which will cure in 15-20 minutes, to the 1:10 used here which sets in 1-2 hours. Thinning the Oogoo didn't appear to affect it's cure time, but I would guess that it would affect the shrinkage. Here's that link.
https://www.youtube.com/watch?v=E_IOqxds130
Don