Swimming pool acid work well to remove the rust, it works quick so you have to pay attention, and then neutralising it with baking soda.
After all the rust is out make sure you keep your tank full of fuel.
The liner is a big job and fails easy, if you going to spend the money rather take the tank to be professionally lined then you know it has been done properly.
Otherwise you can look at replacing it with a plastic tank, I saw one on ebay the other day.
1983 Honda ATC250R
1983 Honda ATC200E BIG RED (sold)
1984 Honda ATC200X (sold)
1985 Honda ATC250ES BIG RED
1985 Honda ATC70 x 2
2001 Honda TRX90
2004 Honda TRX300EX (sold)
2008 Honda TRX250TM
1976 Suzuki RV90M (sold)
1967 Vespa Primavera 125
1958 Massey Ferguson 35