Better to go the metal gears but a well set up upper end nylon gear servo will do it. Look along the lines of a standard sized servo with a minimum of 48 oz. in. of torque. The more prescise and reactive you like to fly will determine just how fast and accurate your throttle servo will need to be. A sport flyer can get away with a standard speed, standard precision servo. Similar to what is included with most radio systems. Someone flying low and wild 3d, or precision, will usually end up wanting and using a faster precision servo. They find they don't like a lag in throttle response that impacts their other control imputs during a maneuver.

The more time and care you take in setting up the engine mounts and throttle linakge/geometry the better the chance of using a nylon geared servo. Never, ever use small servos intended for low vibration electric flight with ultro low torque on a gas engine. They get shaken to death in short order. As far as servo quality is concerned, consider; Once the throttle servo dies your plane became a heavy glider with a far less than ideal glide ratio. Are you good enough to always have the altitude and position needed to make it safely to the runway once the engine dies? Base your selection on that basis and you will be off to a good start.