KISS!
If you're going to bolt it to a sleeper (railroad tie, in American) I doubt you'll spoil the aesthetics with a fuel tank mounted on angle iron bracketry for a gravity fed system.
Use a Jerry can, a bronze BSP fitting brazed into the bottom to take a ball valve will make filter changes easy and bleeding with a 2' head of pressure is simple too. A short length of copper brake pipe brazed into the top will take care of the any return.
Drill a small hole from the underside of the cap in to the 'tunnel' where it swivels to make a dust proof vent.
No pumps, no electric or battery to bother with either and if you want extended run time, double up the Jerry cans and leave both valves open, they'll equalise, and it will save the work of plumbing the return to the second 'can, with a change over valve.
£30 quid if you use new kit, very near nothing if you scrounge thro' the 'come in handy' pile.
Well, that's wot I think.........
Cheers Stef