Electrical Main Panel

I'm not "up" on latest codes (I was never licensed but I repaired a LOT of AC and heating units, and have worked on 208/240 and 460 3 phase equipment)

What I'm finding on the internet seems to "agree" that a 100A panel off a 200A panel is no problem as long as it's fed from a 100A breaker

If that panel is in OK shape and if you can still get breakers to fit, it might be easier to add a second sub "if needed" right there beside the main, to allow more room for circuits