The air filter is accessible by removing the right-hand side middle fairing
Unifilter - Australia
Cost - $50 AUD
Air filters are cheap. They cost around $25 AUD and need to be replaced every 25,000km on a Honda CRF250 Rally. My trip from Melbourne to London would take approx 50,000km so I would need to swap it at leased once. I don't want to bother trying to find one on the road. I could take a spare but I don't like to carry things I don't need. Plus, it could get damaged.
A washable air filter is a great idea for a long-distance adventurer. You can wash it whenever it gets dirty. It's important if you plan to ride a lot of dry and dusty road, which I planned to do. I chose an Australian foam brand Unifilter. You can go with a K&N but I found this cheaper and does the same job.
You need to carry air filter oil. You can get small bottles from remote control car manufacturers. They use the same foam filters.