Thanks Jon, Clay and Scott. Based on a re-reading of the 1967 version of the Instruction Booklet for my 1966 Nagra III, my fears were overblown. It seems that the heart of the rotor portion is a large permanent magnet "fixed in the assembly by a left hand threaded set screw. The winding surrounds it. To remove the rotor, after first having unscrewed the center screw (normal right hand thread), it is necessary to introduce a rod 3mm in diameter into the hole in the rotor between two wires." The manual goes on to describe how the magnet can be unscrewed and the entire rotor removed from the machine into a "keeper" tube of 40 mm in ID and a minimum of 60 mm in OD.
So the rotor assembly detaches from the shaft protruding from the motor side of the encoding wheel (which is described as a "phonic" wheel). So the phonic wheel and its shaft stay connected to the capstan and the clearance to the related read head are maintained. What one needs to do the repair I need is a NOS motor, which I may be able to get, and a keeper tube which I can have made locally. While the motor outer shell and brush assembly are disconnected, I can pull the mounting plate to replace the belts and clutch felts (assuming I can find some felts). So I appreciate your and Clay's comments and will be touching base with the various sources that have been suggested. A mixer friend has an alignment tape and a couple of scopes and signal generators as well as a known good 1,000 Hz tape and a cycle counter if we need to dig further into the innards to correct timing beyond what the flutter wheels indicate.