Got some bushings from ebay, advertised as being suitable for ACS trucks etc.
Certainly look the part, 1/2" tall, red.
They are rock hard however, might be okay for the displayers out there but I want my ACS500's to turn. I'll see how they are once they are fitted.
I've spoken to my friendly fastener supplier to see if he can get me some new kingpins.
Hope to get this up and running soon.