Fill the tank with hot, soapy water using dishwashing liquid or laundry soap. Runs it out good, then pour some Simple Green, Formula 409, or similar degrease in it and swish it around. Dump it out and let it dry. This will get rid of a lot of the gunk in the tank.
Now plug the fuel outlet holes and vent. Then carefully mask off the entire outside of the tank with plastic, as the next step will eat off the paint.
There are two ways to do it: quick and dirty, or permanent. With a restoration, I would go for the permanent fix.
Order a Kreem Tank Liner Kit from JC Whitney or see if you can find one at AutoZone/ Pep Boys. This is a three-part kit. The first step is an acid wash for the inside of the tank. You dump in the tank and put a couple of layers of plastic bags in the filler hole and force on the gas cap. Let it sit for 24 hours, swishing it around every now and then. Dump it out, rinse with water, and dry with a hair dryer or compressed air.
Then you dump in part two, which absorbs water and coats the inside of the tank. Swish it around for a few minutes and then dump and dry, but not with compressed air, which will leave water behind.
Step three is a milk-like liquid you dump in and swish all around to coat the tank. Poor out the excess, and keep swishing the tank around until it all dries so it doesn’t puddle. Let it air-dry overnight, and repeat with a second coat the next day.
BMW coats their tanks like this, as does Harley, but Jap bike tanks are just bare metal on the inside, so they rust out. Your tank is now rust-free and lined so that it will never rust again.
The quick and dirty way is to get some phosphorous solution from the paint shop or hardware store, like Naval Jelly or rust remover. DO NOT get the stuff that turns rust into black stuff; get a phosphorous solution that will eat away rust. Fill the tank and swish it around every now and then to keep the insides wet, and in about 24 hours rinse well and dry. The rust will be gone, but it will of course come right back.
