Your best bet is a bigger heatsink. Or if you havent already, cut a hole in the windshield of your body in front of the engine.
A bigger air filter sill only help IF the current one is restricting the airflow. More air in = better combustion + more air flow, which creates more cooling. BUT the engine can only suck in so much air at whatever RPM its runing at, so if it is getting max air flow with the current filter, then a bigger one wont make a bit of difference.
Hope that makes at least some sense...