Cannondale's have used 1.125 (1 1/8) steerers since 1996. Co-Motion has an OEM steel disc fork that would probably work pretty well on your Cannondale. The dimensions and rake are probably a little bit different (a tad shorter and a tad less rake), but that might not be a bad thing.
As for the water bottle bosses being buggered up, you might just call Cannondale tech support to see if they have a recommended solution. I think most bosses are bonded-in on aluminum frames so drilling out the old ones and bonding new ones in place shouldn't be all that hard... but that's merely a guess as it's nothing that I've ever done.