There are a few manufacturers that make aftermarket brake rotors that are larger than stock.  The modification is fairly easy with the caliper mount moving the caliper further out from the center of the hub.  You could also go with very high end disks and calipers although that would be a few thousand.
However, I'd consider different/better pads, higher rated brake fluid (probably DOT4), along with stainless brake lines, first.  These last mods are similar to what people do for sports cars to prevent fade.
As others have noted, you can also add a exhaust brake if it doesn't have one and it is a diesel.