I assume this variator is the same one as used in the Colchester
Chipmaster. Parts (as you have discovered) are unobtainable - I got
the last spider bit in existence 10 years ago.
A VFD works well on them - I know of a couple of Chipmasters using
electronic drives. Mine is making a horrible noise today, so I suspect
the end is nigh, and a VFD and new motor will be a late Xmas pressie
soon. You get the advantage of exact speed control, RPM readout, ramp
up and down for speed, etc
Geoff