That awesome wrench, I don't know much about whizzers but just know about the belt drive and available rear sheave. Didn't know they had a special clutch. I'll have to look at one.
Not to go into a history lesson but there are more centrifugal Whizzer clutches that have been made than I will mention now
There have been several different gear ratio centrifugal clutches made by Modern Whizzer over the years 50mm, 70mm and 90mm
Modern Whizzer even made a CVT Whizzer The Vintage Centrifugal Clutch was a 2 speed called the BI Matic.
Max Torque Clutch even offered a Centrifugal clutch for the Whizzer but only a limited amount were made thru a special order.
I Have a Max Torque Clutch in my collection and it's the best made. But I use the 90mm 2005 NE-5 centrifugal clutch it has the best overall performance and top speed. If I lived in a hilly area I would use the 50mm centrifugal clutch
By lightening the Clutch shoes in combination with the correct RPM springs the Whizzer Centrifugal clutch can be fined tuned.
A Whizzer slip (belt) clutch control arm can be used in combination with the centrifugal clutch allowing the centrifugal clutch to be disengaged by slipping the primary belt. But it would be of no advantage and not necessary.