There are suppliers of small motors and also spares for more common turntables.
Shaft diameter for the belt crown/pulley is one parameter. Synchronous RPM can be got from measuring pulley diameters and going for the nearest integer number of poles.
My boss recently got a new motor for his Dunlop turntable... Farnell and RS are a good starting point.
AR certainly didn't have special motors made for them, they'll have used standard parts in standard case size.
If you get one with the wrong revs, you could always make an inverter. Very fashionable in turntable circles
David