I got two synchronous motors (see previous log).
The most period correct model is seized.

The bearings look OK, but the grease inside has hardened.

I used flux remover to get them going almost instantly. Now I am rinsing the rotor in isopropyl alcohol.
The inside looks undamaged - i will just wipe it clean from eventual rust particles and will not give it any other treatment.
The windings measure good.