Cleaning out the loose rust scale in the fuel tank can be done with loose metal material placed inside the tank and then shaken up.
This tumbler was rigged up for the shaking part. ¹⁾
You can find some interesting variants on you tube.
Some have been made from tumble dryers, wheelchairs, electric drills and windshield wiper motors.
This one is made out of scrap parts, an old washing machine drum on a steel axle and a large V belt.

The tank is wrapped in old towels, taped to the central steel rod and covered in cardboard to save scratches.
It tumbles away OK once you get the speed right.

Inside the tank is a mixture of brass nuts, some fish tank gravel and a jar of sandblasting garnet.
Let her roll for around 30 minutes and you can see the rusty debris that came out in the 2nd pic below.
And the inside of the tank is now free of flakey rust particles.

Homemade tumbler. 2)

³⁾