If the ally tanks are custom fitted into your hull
, I'd be inclined to refurbish them and, to counter the wife's concerns, coat the inside with an approved sealant
The rotomoulded plastic ones linked are 'generic' shape and size. Roto moulding means you gotta make the steel
mould first, then pour in the plastic in liquid or bead form then heat the rotating mould to form the tanks.The moulds are expensive to make so not a good process for one-off tanks, unless you get lucky and a 'generic size' exactly fits the space available.