Two schools of thought on Zinco's and this is what I go by:
Thought 1: If the breaker failed for any reason other then time in service; meaning arcing, major fault etc., then replacement of the breaker is fine. I always look at the buss bars to make sure there is no discoloration or pitting in them and they look servicable.
Thought 2: If the breaker failed due to arcing, over heating, other other calamity, then I would consider replacing the panel. IMO and experiance (IMO&E?) Zinco buss bars can be delicate creatures and it is possible that there might be a problem with them.
On the whole, Zinsco panels are ok. I personally do not like them because of my own reasons, but as long as they were installed by a competant electrician, they should be okay to work in and replace breakers in until you decide you need a bigger service.