The original problem is the weakness of the jamb. The crack occurs due to stress flexing. The best repair is to weld up the present crack and add a stiffener plate behind it. No need to tear everything apart, just reinforce what is there. This is a design flaw that only shows up after time.
By "still good" I mean there is no crack to weld. How does one add a stiffener plate behind it without tearing everything apart? Is it accessible from the inside? Just trying to make sure I don't have an issue after the resto.