Can I replace a broken toggle on an NS / NSX circuit-breaker or switch.

No, the toggle on the breaker frame cannot be replaced as the toggle is integral to the breaker frame.

The breaker frame needs to be replaced.

However, the toggle extension (4 to 5 cm long piece screwed to the toggle) can be replaced, but only if the toggle thread is not damaged.
 
If the thread is damaged the breaker frame needs to be replaced.