Using your existing 3 speed control box you should be able to run the 2 speed fan and resistor.
Can't remember exactly but the Revotec fans that Jules supplies are two wire motors the same as the 2 speed fan motors
IIRC the purple wire is not utilised from the 3 speed control box and it utilised the resistor then on either the Red or grey but not sure on which.
Fitted one just the other week but the customer retained the wiring instructions
Russ