The previous owner had sort of modified some carriage bolts to mount the tires. They didn't work well but the idea was good.
I spent some time with a grinder and did some more modification.
The middle bolt is the unmodified one, the two sides are with some grinder work.
They fit snug in the rim now.
I also had some damage on the rim that needed some attention.
This area on the rim was badly beat up. A little work after I get a new Dremel should have it looking like new.