The Tornado fork is a Marzochi RAC43.
ferret has spoken to Marzocchi UK, and they will supply the seal direct. Apparently they list the oil seal for the RAC50 fork for GBP 7.50.

gwensp, 10 Mch 2011, reported that the seals for a TreK are the same as found on a Guzzi Stelvio, and can be purchased seperately. Benelli only sells them in the full refurbioshment kit. I'm not sure if the TreK uses the RAC43 or RAC50 fork.

While on the subject of the Trek - kicka666, 13Nov13: "Oil seal size is 50x63x11 (2 springs)"

bennyandthejets (Ian), 30 Oct 2011, reported that the (RAC43) seals are 43x54x11 TC4's. Simply Bearings.co.uk have them in stock at GBP 4.20 each ex vat.
They are described as "43x54x11TC4 Metric Fork & Damper Oil Seal with Extra Internal Sealing Profile & Raised Outer Sealing Band on Outside Diameter". These are NTK TTO seals.

icemonkey, 1Apr14: "Apparently the same size as a Yamaha R1" (edit: unconfirmed)
icemonkey, 11Apr14: "43x54x11 fork seals. Dust seals are not marked but come together if you order from marzocchi uk. Fork bushes were surprisingly worn for 12000kms from new, they come from marzocchi uk also...43x20x1.8 & stops kit. p.s 10wt fork oil is a hell of an improvement over 7wt." (edit: Alex a Maniac Motors concurs)