I am sure this is old news for most Monte owners but I have just fitted a new clutch master cylinder using one for the Fiat X/19 or 1500 which is identical to the original. It was fitted to a number of cars I think. I ordered it as a BCM101 Borg & Beck cylinder - £27 from Carpartsinmotion on a well known auction site, but that part number or 4337695 will bring it up on many sites.
Parts are getting hard to find and expensive so I thought this worth sharing.
Its only the third one I have needed and I have had the car 38 years, so can't complain.