Your best option if it were in a house, would be to replace it with a modern plastic tank.
Yes you can cover the tank, but you are still collecting the dirt, bird dropping from the roof where the rain water is collected from. No one in their right mind drinks stored water from a tank, even if that tank is in a house and covered. The only sensible (for most people) source of potable drinking water, comes straight from a cold tap, fed straight from the mains.
Water goes down your gullet into your stomach which produces stomach acid and where mostly everything is annihilated. If a bird poos on a roof and a little effluent from it dribbles down the guttering into a 100+ litre tank I fail to see how this is going to harm you. Covering the water is essential I would agree. I've seen birds using them as baths and rats sometimes running along the edges and peeing in them. I don't see the danger of a bit of dirt. Keep in mind the water is boiled before use when I make tea.
I feel there's an unreasonable fixation on the drinking water side of things. I've done it a few times and haven't died lol
Furthermore, what about the contaminates and all sorts of weird chemicals you may ingest through tap water? What about fluoride, heavy metals, chlorine, liquid plastic.. all of this junk is in our drinking water anyway lol
Anyway.., thanks for the suggestions everyone. I may sell these afterall and use the money to buy an IBC instead. The hope was that I could give these tanks a second life but I may use the ones that are water tight and sell the ones that aren't.