I highly recommend covered2go
http://www.covered2go.co.uk/travel-insurance/ I have used them on a number of occasions for biking trips. If you opt for Activity Pack 3 it specifically covers 'off road motorcycling up to 250cc'. If you are not 'off road' you are automatically covered for any bike you are licenced to ride without opting for the additional Activity Pack. They also cover trips up to 18 months duration.
On the other side of the coin,
avoid Dogtag http://www.dogtag.co.uk/ I recently insured an off road trip in Andorra with them rather than covered2go because I didn't want to have to lie about the size of bike (300cc). I had a crash needing hospital treatment and their so-called 'emergency team' were useless; they actively prevented me getting treatment for a number of hours while they struggled to get their sh1t together. Not something you need when you are smashed up and in A&E in a foreign hospital.
The trip sounds great - have a good 'un.
Colin B