If it is the timing gear, and it does seem likely, remove the plugs and take off the dizzy cap. Hold the rotor while putting pressure back and forth on a wrench on the crank nut. A good fitting socket and long extension helps. You should be able to feel the connection through the gears/cam/gears with very little play. The steel "star" centre comes loose in a variable way so do the test at several places in the cranks rotation. This test isn't foolproof but should show up any bad play.