Polyurethane is easily machined so drilling out the bush shouldn't be a problem. Use a very sharp drill bit with the drill on the highest speed setting, freeze the bush as well to make it slightly stiffer and easier to drill. Your ARB must be 13mm unless someone has changed it for something from a different car, although I can't think what! It's definitely not from an HC range car if it's more that 13mm.

"When you have a Firenza like mine who needs any Minis?" Copyright David Maxwell 1979.