The servo case looks like this internally:

With spacers and seals and valve like this:

The following three pictures are shots of the dual acting diaphram in the servo.
For the ABS system to work properly, any replacement servo would need to be dual action (ie, a potential vaccum on both sides of the servo diaphram). A source for diaphram seals has not been found. No known supplies exist off the shelf, but remanufacture remains the only option as far as I am aware: