I’ve herd of people putting either small rocks or ball bearings with a little oil and shaking the tank to knock all the rust loose, then dump the rocks/ ball bearings and rust out, put about a qt of gas in and shake that to dissolve the oil, dump that. Then you can either install and fill the tank, and keep it full so no new rust will develop or you can treat the tank with this stuff called “Cream Coat.” I’ve used it on an old Harley tank for a sporty I had a wile back. It’s a two step process. Fist step etches the metal inside the tank. The second puts a Teflon like coating in the tank. Just follow the directions on the kit, it’s pretty easy. You should be able to pick it up at a bike shop, probably not a Harley dealer, more like a custom shop.