Dore-Westbury Gearbox Question

Hi there all, I have recently aquired a part finished Dore-Westbury mill which I believe it to be the Mark II version although the drawings supplied with it do not indicate Mark II anywhere. Included in the box of bits to finish off the machine are some 'straight cut' spur gears for the epicyclic reduction gearbox. I seem to remember reading about these gears being noisy and a design change was made by Model Engineering Services to 'helical cut' gears running in an oil bath. Does anyone know whether I can just substitute helical gears for the straight cut spur gears? and what changes were made to allow the gears to run in an oil bath? If anyone has the drawing details for the revised gearbox I would appreciate a copy (cost of copying + P&P provided) Many Thanks, Roger