i've been debating how to solve this problem as well.

the Kreem product looks like a possible solution. i've also heard of inserting bits and bolts inside the tank, with diluted acid/water, agitating the tank and rinsing it out. then coating it with a similar Kreem product.

http://www.motorcycleanchor.com/motorcycle/how_to/mc_tankrust.html
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MC28 sp