Hard to know why it was lined in the first place. Could be that it was done years ago as insurance on a new paint job and didn't really need it. So if there are no leaks, I would run it as is...make sure to add in-line filters. Going back to my previous post, I despise liners and if possible, avoid them. However, if there are leaks, and you can't braze them, then a liner may be the only option.
My method To prep the tank for storage and to keep it from rusting just after cleaning out with evaporust or whatever de-rust mixture used, I'II rinse the entire tank with water, drain as much as possible, then pour in a cup or two of acetone (or denatured alcohol) and drain...this seems to help get the remaining water out. Moving quickly, I blow a little compressed air, use a heat gun to warm up the tank and help evaporate things faster...and then I mist WD40 all over the insides. I have a tank right now sitting on a shelf that's still rust free 3 yrs later. I use WD40 since I can spray it and it mists up covering everything...but after reading Jim's process, I like the idea of acetone mixed with oil (I'd probably use 2 stroke oil), seems like a solid idea and saves a step.
If you have to add a liner, as I mentioned, I bought Por-15 and was going to go that route on my current tank project. I have heard great reviews...but with a failed liner, Caswell's states that it works on/over failed liners including Por-15, and I also heard great reviews about it. I like the 2 part epoxy concept...so that's the route I took.