The Mk1 & 2 use the same filter as the XR400. This can also be used as a replacement element for the Mk3 & AR. Various makes are available but stick to a filter that has two densities of foam and keep it well oiled with air filter oil. Filters need cleaning regularly to maintain engine efficiency and it is useful to keep a spare for use when you are cleaning one.

TwinAir make a plastic filter cage (for the XR400) that improves air flow and is lighter than the original Honda one.

Sticking a K&N filter on and getting rid of the airbox is not recommended and those who have tried it have never got the jetting sorted.