Ideally do both - keep the gears (or much better replace them with timing belts) and then add the induction motor. The power output from an induction motor will reduce with speed, when driven from a VFD, so it's best to keep some form of gearing so that you can still have a useful amount of power at low speeds. The other option is just use a huge motor, but that's often not practical/sensible/economical.