stringers

Are they rotted? I wouldn't worry too much about wider and certainly not about the height along the length - As a matter of fact, I wouldn't worry about them at all unless they were rotten and/or delaminating. If you do have to replace them, the dimensions you had worked fine for the past 20 years. If you want to beef them up it couldn't hurt; but it would require more grinding since you'd have grind everything totally flat. If you rebuild it as it was (what I'd do), it is a simpler process since you already have the pattern and you know it works fine - plus you could fit them right back into their original position without any error - because you didn't grind the base of the fiberglass casing all the way down.

What's more important is how you bond them to the hull. There are lots of threads in here on this subject. But what's really most important is: Keep it simple.