If the cylinders are rusty or if there is any rust at all visible through a spark plug hole, then the heads need to come off. [/story]
And if the cylinders are pitted from corrosion, then it won't last long even if you do get it to run in that condition. I have an '86 Kawasaki GPZ600 on my to-do list right now that has a cylinder so rusty that it was stuck and sounded so gritty when it came loose that it was audible through the cases. I got it fired-up anyway, but that cylinder was deteriorating rapidly in the few minutes I had it running. New pistons & cylinders are already on-hand.