I know the problem, CB900 has same issue. You will probably have to take a bunch of measurements and find centre section from a different bike then cut/weld bits. It's too long ago since I worked on KZ750 twins, but I was working at Suzuki-Yamah dealers at the time. Luckily there are not too many different diameters, you may get lucky and find a replacement from Suzuki or Yamaha, they tend to have some parts that interchange even though they 'shouldn't'. Maybe GS1000 or 1100? You may nee a different bush running on crank though