Worked on it again the other tires are down to, looking at some solid hand cart wheel tire assembly, but has off set hub, can't see how that would matter plenty of clearance. but definitly going to go with no flat tires.
Most of those type of tire/wheel assemblies are available in versions with different hubs (offset, centered, wide, narrow with a variety pack of bushings included, etc) so i would keep looking for the closest thing as it's probably out there.
Befco puts a great mower on lousy tires. All 4 of mine went flat on my C50 after 2 seasons. I looked at foam filling but was able to find run flat replacements for the same price.