You can order a stock air filter through GPX USA

They also have a funnel web filter available.

If you choose to branch out, the KTM 450/690 RFR filter should work fine.

• For KTM Air Filter Cleaner Element 450 Rally 660 Rally 690 Rally 58506015000
• Air Filter Twin Air 01 58506015000
• KTM 450 RFR 2015-2018 AIRBOX SUPPORTS KIT AND TWINAIR FILTER
  • Filter is standard fit but plates are slightly smaller so require some modification (not really worth it).

LtoR - stock, twin air, eBay, funnel, xl filter skin. (grommets appropriated from stock).

First view of the EURO 5 airbox assembly: