The classes should be defined in Ships_init.c under the 'refShip.Class' lines. The text describing ships as "class #" in-game would be elsewhere, though.
I would advise against changing the class system into a rating system, mind. We tried it for the Build Mod, and have since completely reworked it into a 'tier' system based on crew size.
Ratings work fine for navy ships, but many vessels are merchants, so applying the rating system to them will not work so well.
What we tried to do in the Build was use three separate class systems simultaneously: naval, merchant, and "balanced" classes, with the former based on ratings.
It turned out to be too confusing, having the same numbers meaning different things, so instead the tier system was a good trade-off for all vessels under one roof.
You can experiment anyway if you wish; I just wanted to let you know how it worked out for the Build Mod.