[rotorwrench]

I haven't seen any spark plug repair bushings specific to flatheads but I remember the ones we used on the Continental aircraft engines. They thread into the plug boss and are pinned with a single pin inserted in a drilled hole after torque set. I just don't remember what the outer thread was. Where there is a will, there is a way. Some might weld them up & remachine but that can mess with the heat treat of the head some and may cause cracking later. If there was a bushing sized at 18MM thread on the outer diameter, you would be back in business pretty quick as long as a 14MM plug would fit the bushing.