Well, I put it back together after super-gluing that part back on as best I could (sorry Ron!) and after cleaning and re-oiling it its running better than before. Most noticeable is when stopped the armature spins twice as long as it did before. The speed is better, and while my present spring is a bodge consisting of thin solder wire wrapped tightly round the motor shaft, its running OK for now.
I guess epoxy would have been better, but if the motor fails I guess I will replace with one of Stamanns repro models, thanks for the advice folks. I will upload some pics of this so it can serve as a guide for someone else who wants to disassemble and clean their TT motor.