You're kinda stuck
The issue with the style of panel you have is that the main breaker, instead of being bolted to the busbars the way modern main breakers are mounted, is plugged onto the bussing system. However, instead of using the same bus stab system as the rest of the panel, Challenger used a stub of Zinsco-style bus to mount some of their 200A mains, probably because that was the 200A breaker design they could get rights to.
As a result of this, there just aren't good replacements available short of wholesale panel replacement or gutting the panel and fitting a BR main breaker retrofit interior, especially given that your existing main breaker has a staggered side-exit lug configuration. (As an aside, Milbank still makes 200A Zinsco-style main breakers, but not in the side-exit configuration that you have.)
I would simply keep a close eye on the breaker for any symptoms of trouble (overheating or malfunction) for now: the Zinsco bus system was not a good performer at the end of the day, but I'm not aware of any way to retrofit say an Eaton BW main breaker to these panels.