I cant advocate the use of a tank liner. Over time those flake and gum up the whole works. Strip the Kreem from inside, seal tank and fill with Milkstone Remover (available at Tractor Supply Co, $13/gal) and mix 1:3 with water (one gallon MSR to 3 gallons water). Fill tank with solution, let sit for 2-3 days, drain, rinse well with water and dry. MSR is phosphoric acid, which will not only dissolve the rust but also chemically convert the steel to a grey rust-proof coating. I did a tank and let it sit in my garage unsealed through humid summers and winter thaws, and that tank never rusted one bit. Best part is it's cheap, won't flake off, and will never rust.
As for your petcock, get it brazed or repaired properly. Then you never have to worry about it again.
Wear gloves and glasses when using the MSR. It will burn the crap out of you. Also do not get it on your paint, it will fade it. Save the MSR after doing your tank, it works wonders on chrome.
EDIT: I forgot to add. Check your petcock to tank seal. I had mine leaking on my Yamaha RD and here it was a bad job with the paint, a small run was where the oring sat against the tank and it started to leak. I sanded it flush with a sanding block and a year later it's still not leaking.
