A high-flow panel filter is a must if you're retaining the standard airbox but want to make the most of what you've got.
 
These quality filters can flow 30% more air than a disposable paper filter and offer a washable, re-useable filtration medium.
The foam composition is capable of holding far more dirt than a corrugated oiled cotton or paper design without the subsequent restriction to airflow.
 
These are my personal favourite; they're well made and incredibly easy to clean, improving performance AND saving cost in the long term.
 
Handmade in the UK and supplied with a Pipercross lifetime warranty.