The bikes are coming with a standard paper filter (P/n 60306015100). A replacement foam air filter is available for dusty places. P/n 63506915000. Feedback about this foam filter is less than stellar, it needs grease to fill the side, and is hard to put in place (as all foam filter this filter needs to be oiled).
Alternatively, KTM offers a foam pre-filter system, replacing the filter cover and the two trumpets (p/n 63506915144, €60). Due to its size, it is not compatible with the dual seats, and not compatible with the alarm.
For sand, this prefilter can be topped off with a pre-pre filter (p/n 63506922000, €20).
Rottweiler offer two different replacement systems for the air intake. The first one is similar to the KTM prefilter (but predates it by at least a year!) as it replaces the filter cover and removes the snorkels. It’s called the PowerPlate and cost around $90.
They also offer a complete replacement airbox, with a foam filter. Available in carbon (Rally Edition Full Intake System $650) or rotomolded plastic (Pro Edition Full Intake System, $350), in different finishes and colors. They claim up to 10hp gain by just installing it.
Unifilter has foam filters available. 100 A$ a set of two. Those are the EOM fitments in Australia. With yellow and green foam in a reusable metal insert. They also offer socks that can be set inside the air intake for additional protection (but those are intended for off-road only). There is a lot of info about unifilter on this FB thread including a comment from the designer of the part, Myles Gooch, explaining its proper care :
[…] I designed the Unifilter Australia Filter. You don’t need to grease the foam inserts. You just need to make sure you insert them correctly. Inserts the green one first. Make sure it’s fits nicely, then insert the yellow layer. With the 790 you need to treat it like a regular dirt bike when riding in dust. The location of the intake is similar to a dirt bike. You should inspect the filter every 150-200km when riding in dust. We supply the extra foam insert due to the intake setup drawing lots of dust from the back wheel. This allows you to change it easily when the conditions require it. Always make sure you use a quality filter oil. Avoid spray on filter oils.. always use grease on the seal and you will never have an issue. You can use the sock style pre-filters when riding in dust to increase the service interval. These should be oiled to work effectively too. Cheers
Guglatech offers a Rally raid air filter that works with and without oil.
Other companies do offer replacements: BMC Air filters for example. Leave a comment if you find more.