Hands down, the Cam-type servo saver by HPI (part number 87197) is the best route. It upgrades the steering assembly to have a much better servo saver. While you're doing that upgrade, you might want to toss in some 6x10mm bearings (4), in place of the plastic bushings.
Now, as for a good servo, I have a JR 650M in mine, but the Hitec 625mg or 645mg is a great servo for it.