Unfortunately all these parts are obsolete and I know ofnoyhing that would work as a patch up.
There is nothing that is a good change over for a Watermill because it has left hand inlets the only shower on the market that I know of that has left hand inlets is the Supajet 100 and 200
http://www.shower-warehouse.com/new-showers/power-showers/supajet
Not a very good internal view
http://www.showerdoc.com/shower-spares/supajet/SUPJ-PARENT-1-Supajet-100
This has left hand inlets but uses a mains powered pump therefore the unit will need to be earthed.
Hi Trevor
Unfortunately the motor and pump units for these are obsolete.