Hi Richard
So far as I am aware there is no circuit diagram or other information concerning the Erskine drive units out in the wild. Which is a pity if you have problems as the only repair service I know of is an occasional advertiser on E-Bay at around £90.00 exchange.
Circuits for the American native Bridgeport drivers are around but these are inferior to the Erskine unit. Much simpler and therefore less costly to replicate tho'. However its a standard 90 v DC brush motor so pretty much any decent modern driver should be up for the job. Something adequate can probably be found around the £100 mark if need be. Although perhaps a little more fragile than the Erskine, which generally speaking are remarkably durable and long lived, the modern microprocessor controlled units may be even better at the drive business.
If you plan to do the two axis power feed conversion I'm afraid circuits for the switching relay unit which selects X or Y traverse are also unavailable.
Worth cleaning out the motor as, over the years, copious quantities of carbon from brush wear accumulates. The job is, unfortunately, second only to cleaning out the sump in the general mess'n getting real filthy stakes. Worth the effort as a good scrub and new brushes frequently restores an apparently dead motor to life.
I have the mid 1970's Adcock Shipley wiring diagram in PD format if that helps. High resolution scan so it blows up really well should you need a better view of the section you are working on with room to scribble.
One age related hard to find fault concerns the overload units on the contactors. Symptoms are occasional tripping for no known reason and failure to auto reset despite the control being set to auto. Proper cure is remove, pig of job as there is little room, clean, contact lube and exercise the auto / manual switch a few times. I've gotten away with a good squirt of contact cleaner / contact lube and selector exercise so far. If I have to delve seriously the old contactors are coming out and a modern DIN rail mount set going in.
Clive