A master cylinder shouldn't fail after 3 years, I would be trying to work out why.
Are the metal flakes from where Morgan have modified the cylinder (eg enlarged fixing holes) and then stamped MM on cylinder, or is cylinder too short a stroke for application & bottoming out?
Any 5/8" cylinder with sufficient stroke will work with appropriate pushrod & mounting flange mods, but perhaps you don't want to be modifying a 3 year old car - unless Morgan got it wrong!
For UK members a range of master cylinders are available from Car Builders Solutions, a quick eBay search shows that 5/8" cylinders were even used on German cars including Audis.
