If the problem is just on the one lightly loaded circuit, I'd rather double up via pigtail splice to another lightly loaded circuit. The poor contact between the breaker and bus may cause problems under load, even if it seems ok now. And I wouldn't rely on the owners promise of changing the panel in the future. Too many things can happen in the meantime.
BTW, is there a such thing as a "good" Zinsco panel?
