Hi there moggyjim
John Cruse at the Mower centre, Hailsham, East Sussex (01323 842477) has mad 700 clutch cables for me in the past, not sure if he has a pattern, as I have always sent him an old one to copy. If you have not got a pattern, I may still have one here you can borrow, let me know.
I have a Howard 352 with the jockey pulley system, it works very well. I think the guy with the ‘Howard rotavator spares’ website may still market a centrifugal clutch for the 350, I think he used to sell them.
Chris