Is a product used for that purpose. May not be available anymore. 

The base element is Phosphoric acid,  (30%) usually. Available on line. If your looking to dislodge heavy deposits couple of bags of old nuts and bolts and agitated  in all directions to knock the deposits off. If you believe a tank liner product has been used put a litre of acetone in there a. Leave it for a week ensuring it wets affected areas. Both products require controlled handling so read up and protect your self and your machine

J

Then Tank Care Products will do what you want.
Others may also, but these are the only company I've seen regularly advertising they will derust and apply liner for you. 

www.tankcareproducts.co.uk

Regards, George 

Vinegar will remove rust, also Coca Cola.

Fill the tank and leave for a week or so.

I used to keep a container of vinegar in my workshop and left heavily corroded brake discs in it.

Came out like new.

I found steel tended to flash rust after being treated with vinegar - i use milk stone remover now (phosphoric acid) and neutralise with bicarbonate of soda.
dan 

electrolysis also works well. 

Whilst there are many types of rust remover available; acid-based, alkaline-based and water-based;- one that is relatively safe, cheap and easy to obtain is Citric Acid. My choice at home is the food-grade type, which is great for cleaning lime deposits from kettles, taps and any bath or kitchen metals. You can vary the dilution, so ideally it would be useful to be able to test it on a rusty piece of metal.  It's also used in wine and beer-making!

A well-known problem with mineral-based acids is flash-rusting immediately after using.  It's down to the moisture in the air.  When steel is chemically clean, it can rust microscopically almost instantly, unless the environment is very dry - low humidity.  My theory is that this is the problem with applying linings to tanks.  Microscopic rust, which just grows and debonds the lining.  With all the horror stories I've heard, I would never line a tank!  I'm considering trying zinc-plating, but electroplating produces explosive hydrogen - not such a great problem when surface plating in the open, but a petrol tank is a very confined space!
 

I was discussing this problem of rust removal from a flat tank Norton at one of the shows and a knowledgable gent explained how he uses electrolysis to clean the tank. 

Try typing this into google (or other search engines!) to find You Tube guidance:

Removing rust from gas tank with electrolysis

A tank that is about 100 years old is a delicate thing, so putting stones or nuts and bolts in to knock the rust off is likely to create even more holes!

Once it's clean you have to choose which lining to use.  NOC Shop does sell one product, Tapox I think?  I was recommended POR15, which is sold by Frosts.
Good luck with this.

Frost works for me every time. Still good after 10 years.

Try Bilt Hamber DEOX-C it worked a treat for me. Comes in powder form which you dilute in water, it's environmentally friendly and safe to use, no awkward disposal problems. I've never had a tank lined so can't comment on that.