Kev,
while it looks like the motor/encoder is no longer available, the encoder is widely available ($40-60).
The encoder is really not serviceable, The old X motor/encoder was in a parts box, It took the old encoder apart but damaged the optical disc. having it apart, I don't think there is any way to get it apart once installed without a high likely hood of damaging the disc.
There is a silicon dust cover available for that model encoder from the manufacture.
I wonder how many $500.00 motor/encoder assemblies were swapped out do to a dirty encoder? That a dust cover could have prevented.
I am guessing that is why they went to an "encoder strip",, serviceability.