Another update: The stator has been welded. That was not an easy task because of the special cast iron alloy it's made of. Also the shaft had to be cut, otherwise the upper motor plate couldn't be removed. It turns out that the shaft has a slightly conical shape! The shaft bore on the rotor has the same negative form. I have never seen this before. 70's good old engineering...
It took a long time and quite a lot of drilling, machining, welding, grinding etc. The siren is now fully functional. Painted of course in its original colors.
I own: One complete SiraTone, another SiraTone control module, a 1600W MC, 2 old ATI controllers, 2 round ATI horns with drivers, a ATI-DSA 4 thing, a Federal Signal FC, AR timer and Siratrol, and a small CD&F.