On the fuel issue, you can get an oversize tank for the Montesa. Also there have been some tank bags floating around over the years that let you store the extra fuel bottles on top of the tank. Not sure of the brand or where to get them.
I went to using a small backpack with a waist strap for a few dual sport rides, and if you get a decent one (I got a Kelty Torrey, built in drink bladder) they are pretty comfortable. You can put plenty of camp stove fuel bottles in one of those.
There was a Monty on ebay a few months back with the big tank and SA; I think it was in Colorado.