This highly restricted airbox has only one possible solution to get some extra power , a power-up kit . Make some extra holes in the airbox to increase airintake surface and develop a airfilter to clean the air of these extra intake holes. With this done you increase the air intake opening with at least 50%
On both sides of the airbox you need to drill holes according the included pattern drawing (with each filterkit).
The power-up kit will have part.# MC-025-13 and includes two filters with patern drawing and screws.