Any mild acid will work to get rusty deposits out of an engine. The one that's specific for rust is Oxalic acid, which used to be the active ingredient in the old DuPont powdered cleaner. With any sort of acid flush, it's imperative to follow up with a box or two of baking soda dissolved in water to neutralize the residue, followed by a clear water flush. About 3-4 ounces of the "muriatic acid" (technical-grade hydrochloric) used to clean up after masonry work will do, but don't use it on an engine with aluminum cooling system components, and neutralize it well after the flush.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!