Creem kit costs about $30. You can get it online or at most motorcycle supply shops and some dealers. You can use kerosene but I doubt if it will get the rust out. Have you ever seen the bottom of an old paint can that has been setting for several years? That is something like the bottom of your gas tank. The old gas, oil and moisture combine to make a glue like or varnish in the bottom of the tank that is difficult to get out. You can try using kerosene and then flush with gas until it runs clean.
The issues are if you don't treat your tank properly the rust will continue to clog up your carb. The rust could also eventually rust through your tank.
Whatever you do after you clean it out, keepthe tank full and the oil/gas mixture will help keep it from rusting and minimize condensation. Drain and clean it if you are going to put it up for the winter.