Back in the day, bias tires were standard issue on Isetta's. Old style bias tires have a slightly thinner profile than modern radial tires. Replacing an Isetta's bias tires with radial tires creates one small problem. The distance between the tire and the front shock towers is reduced creating a situation where the tire could potentially rub the shock towers.
There is a solution for this. I recently picked up a set of David Brower's (aka Rimspoke) aluminum wheel spacers. They are nicely made, reasonably priced, easy to install, and eliminate the possibility of the tire rubbing the shock towers.
Installing the wheel spacers.
Before and after photos showing the increased clearance between the shock tower and the tire with the spacer installed. If you're running radial tires on your Isetta, Rimspoke's wheel spacers are the way to go.