I would say using a Metro servo is bordering on dangerous - particularly if you fit disc brakes at the rear: they require more pressure to make them work effectively, which the Metro servo set up is incapable of generating.
A quip by Patrick about driving his Sportster with a Metro servo has stayed with me throughout - "Trying to brake hard behind a modern tin-top with a servo bigger than its engine can be scary!" - summed it up perfectly!
I do not have any dual servos left, but they are easy enough to order from US - but be aware you will have to pay import duty (5%) and VAT (20%) on the total purchase price.
I have some surplus adaptor plates if you go down this route - and can give you the part numbers of the Dual Servo and appropriate Sierra Brake Master Cylinder - if you are interested, ping me a PM.