Given that I haven't seen one of these setups up close - there are speedo drive parts on ebay.
They may be the correct ones?
Gears - in the US - ebay 300599906591
"Speedo gearbox" - UK ebay 320979240311 - looks like part # 5242 - 38?
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.