Is that a big deal?
theres a very very small amount of rust on the top 1/8th of a inch of the cylinder wall.
I dont even thnk the piston comes up that high but figured id ask.
i dont think that is rust.............i think you are seeing thick brown carbon deposits i had them on my cylinder walls.(where the pistons didnt touch)
get a ridge reamer and cut it off so the top ring doesn't catch on it and break, if you want. You can just put it back together, that's a normal feature of piston engines
Take a little fine emery cloth or a fine scotch brite pad, spray a little wd40 on it to catch the debris and lightly clean it up. If it comes right off - no problems. If it's deep enough so the cylinder wall is pitted, and it's below the ring line - that could pose problems.
(with a rag) wipe out the debris you can, bring the piston to top and wipe it off, then drop the piston again and some more debris may be left behind to wipe out.