Yes, Mike, you can change the complete gearbox. Any other Ariel CP box will go straight in as will the heavier duty BA from the 500s. However I am sure that Neil Baker, Aomcc gearbox guru, details page 2 of the Cheval, will have the necessary parts to make the change to your own box. I had a VCH for some while with the same gear ratios and it drove me crackers so I swapped the ratios.
Editor Cheval de Fer