There are a couple of large tank options, but they are pricey! There was a picture of one floating around on here that had a pair of big baja lights and a 27L Aqualine tank. It looked like it was ready for Baja (which is where the guy took it!).
2014 KTM 500 XC-W
2011 KTM 990 Adventure R
2010 KTM 530 EXC
2009 KTM 950 Super Enduro Erzberg Edition