The fuel tank looks like the
simplest thing, and it is. The temptation is to use a
regular, rectangular shape can, something from a motorbike
or mower. The important thing is to avoid possible surge
leading no fuel pick-up and the possiblity of stalling. A
long shape like this works really well, and fits anywhere.
Holds about 4 litres.
I don't know where the metal cap came
from, but a normal plastic fitting is stuck inside & the
cover provides a labyrinth to prevent mud getting in when
the cap is removed.