I have had some questions from bikers about Biking Viking. It seams like Njall, the operator of Biking Viking, is having problems with his web-provider. At least used to have, earlier this summer. Everything is running smooth at the moment, so send him your questions on firstname.lastname@example.org
Neither KTM or Honda rent bikes in Iceland. Biking Viking is the only company at the moment renting enduro oriented bikes to use in the interior. I would not take the Fazer 600 to the interior. It will be very good on the aspalt part of the road system, but since the interior is only gravel, and at some parts very rough gravel, I would only take a Paris-Dakar style of bike to the interior.
I hope this helps you guys.