I tried to find a large tank for my DR650 last year, the only ones i could find were the IMS tank at 17 litres in white clear or blue, from the USA and the Safari tank at 30 litres in White clear blue yellow red and black they don't always have these colures in stock, from Australia, both will ship to the UK. As far as i know Acerbis only make a large tank for the DR650 models up to 1995.
I have heard of a couple of European companys that make large tanks to order but they are not cheap, you may want to try on the German web site dr650 forum.de [i think this is correct address ] They have some helpful links. Hope this helps Skip
Do the best you can with what you have,
A stranger in a strange land now heading North South East West to.....