Welcome Ludwig II, im digging your artwork. It has some similarities with Big Boy Cycles, here it is just in case you havent seen them, they are very nice bikes. http://motorbicycling.com/showthread.php?t=36428
I know the perfect tank for your drawing,being from England you may be aware of the English built engines "Villiers", and Villiers engines can be found in Villiers/Atco cylinder mowers, however im not 100% sure if the Atco mowers are English or Australian so im not sure of the availability over there, but they are not that hard to find over here.
Heres one of those tanks on an unfinished bike i have