It all depends on the quality of the insulation. Most WW2 stuff used
simple enamel and varnish insulation on the wires-- you probably don't
want to push that much above 50C for very long.

The general rule-of-thumb for old transformers and other things with
windings is: If you can hold your thumb on it for twenty seconds, then
it's okay. Actually, better than okay, the warmth drives out the
moisture which is the number one killer of aged transformers.

Actually the rotor and commutator are what get really hot-- you can
usually tell by smell, if the output air from the dynamotor smells like
varnish, it's too hot-- the varnish is slowly vaporizing, eventually
something is going to short out.