Hi,
I had a later model robson engine and ended up selling it to fund some other projects.The govenor balls were broken and I ended up using a pair of 50 mm tow balls and machining the threaded section flat to fit into the pivot points.Being steel,you could easily modify with a welder and machine to suit.I dont know if I still have the old cast ball or it went to the new owner.Will have a look.
Cheers
Lanzalldog