If its genuinely limescale then any acidic solution will remove it - the trick will be making sure that the metalwork on the engine isn't damaged at the same time. Are you sure it is limescale not some sort of horrible corrosion of the aluminium leg. If you are looking to dunk the leg in something (rather than carefully apply only to areas which won't be damaged) then something relatively gentle like citric acid (from pharmacists?) should be less harmful to any sensive bits.
If it is limescale on the outside of leg, I would be very concerned about the state of the interior of the engine cooling passages.