From new the bike should have an ignition and steering lock so if yours had neither I would suggest a big lock and chain or and a disc lock.
As for any must have bolt on there is not that much available for the bikes in the uk. I few people have ditched the heavy standard tail pipes in favour for lighter after market ones that give a little bit more power also but found them a little loud for green laning. If it doesn't already have them I would suggest a decent sump guard and hand guards?
i had no problem insuring mine with swinton but as with all off road style mobiles the premiums do seem a little high for the bikes.
Adam.
-- Edited by Adam on Wednesday 15th of May 2013 06:23:23 PM
Keep the motor as standard as possible and concentrate on getting the right gearing for the stuff that you are going to ride, plus the usual bike and body protection - bashplate, frame guards, handguards - plus bar risers or taller bars if you need to improve the riding position.
Above all else, enjoy riding it - they are great bikes