Yay, great news. Next the startup. Good luck with that.

If you haven’t cleaned the inside I would use evaporust does a super job any pictures of the inside of the tank you got SPARK. 

Start with Evaporust Mike or electrolosys. Both are quite passive when compared with other stuff.
You can make up electrolosys easy . An insulator to go over the fuel tank filler (wood or plastic) some baking soda and mix it at a dessert spoon full to a litre of distilled water. You then want a 12V battery charger( You can run it through a battery if you like) fill the tank ,suspend a mild steel coat hanger bent to shape so as not to go to earth in the tank,make a hole to pass it through on your insulator,earth on the tank,live on the coat hanger wire, switch on and leave it for 24 hrs. Job done. You can repeat. Then you can use your liner kit to finish off the job much easier.

Wow SHYTED that sounds like a cool idea i think i'd be dangerous at doing lol--but cool idea for sure.
Be good to see a video if any out there. I guess google is my friend.