I can't believe they put that stock servo in there from the factory....well yeah I can. A lot of, if not all, companies skimp on the servos and/or electronics in their RTR cars/trucks. That drives me nuts, not being able to buy a kit and put in what electronics that I like/choose. Looks like a standard size servo that is needed for that HPI. Look for a metal geared servo with more torque.

For a suggestion....Hitec HS-625MG High-Speed 2BB Metal Gear Servo for an analog servo, or maybe step up to a digital version... Hitec HS-5625MG High-Speed 2BB Metal Gear Servo. They are my go to servo for heavier RC vehicles. I've tried a lot of the super low budget servos; with specs too good to be true, and I don't skimp on what steers my cars anymore. IMO those Hitec servos are just superb for the money. You want more torque/speed in a servo...ya gonna need more $$$.

Everyone has their favorite brand(s) of stuff; I'm just throwing out a suggestion that has survived 5+ years of my abuse with only one replacement set of the gears needed.