My house had 100A service, and that's what I fed it off the new panel. The leads coming from the mast weren't big enough for 200A service. It was 2ga AL. Bare minimum for 100A.

I'd be prepared to replace the whole works from the mains to the panel. Do it, and write it off on your taxes. Like Jack said, when it comes time to sell, that improvement will pay off again.