Milners have been supplying filter kits for the Fourtrak at the princely sum of £10 + delivery and VAT; these consist of the oil filter, air filter, and fuel filter.

Anyone buying a kit should note the following, two different air filters were fitted to the Fourtrak, these are essentially the same, except for the locating hole (large one).

When ordering from Milners, remove your old filter and measure the large hole, supply Milners with this information, as often they do not ask. Many vehicles also have filter housings fitted from other models, and may not be fitted with the correct filter for the original engine.
Hopefully, this will prevent problems, which I personally, have encountered.