The genuine S2R 800 filter is 42540101A about £25 or an equivalent Mahle KL145 can be had for as little as £10.
When I replaced my Evo filter, I found that the OE filter was actually made by Mahle, so there should be no problem with quality.
http://www.ukmonster.co.uk/monster/s...ad.php?t=58075
The only difference between OE and Mahle is that the OE filter has a small integral mounting bracket but it doesn't seem to be essential. Velosam recently fitted a KL145 to his 620 without any problems.
http://www.ukmonster.co.uk/monster/s...t=58986&page=3