Dish soap, cordless drill and a length of 3/8” galvanized steel cable.
I find it best to solder a couple places to prevent unraveling.
For the drill chuck end, brush about 3” with flux and solder like you’re soldering a copper pipe. For the cleaning end, come up the cable about 6” and brush a couple inches with flux and solder.
Once cooled, fray the cleaning end out. and start working it into the bottom of the tank with some soap and water. Drain, rinse, repeat. I use a pair of leather welding glove because a wire through hand would suck.
If it’s still giving you rust specs, get a steel wool, the coarse wire type, not the fine stuff. Cut a chunk of it and thread it onto the frayed end. Keep drilling. Forward and reverse. Pulling the cable almost all the way in and out but be careful because it’ll want to coil up on you.
It’s easier if you’ve got a bicycle stand so you can tilt the bike forward. I removed the front tire so I could tip it all the way forward.
I then tip it forward and get a 1/2” or 3/4” hose and use it with a shop vac to suck out as much as possible. I flush it with water and even the blow nozzle on an air compressor in the petcock hole.
Then, after I’ve gotten it as clean as possible, I go through the recommended evaporust soaking and such.
FYI, I spent 5 hours on my last Maxi. I’ve promised myself to not buy rusty tanks if at all possible.
And don’t forget to run a fuel filter.