I have an 049Cox motor I have taken the cylinder off the motor and I have used a brass brush to scrub the piston. I can not fit a brush into the cylinder. Here we have to get into circular thinking. I use a bore brush from a shot gun or rifle or piston. I find that a 12ga fit the .049 just right. I know you are going to say it is to big  Go with me on this. I chuck up a 12ga shotgun bore brush out of bronze with a brass center wire(if you use steel center wire it will scratch the bore.) it fits the 049 Cox just right I do use my cordless drill to hold the brush.I turn it clockwise going in and out. by working it back and forth with and with out alcohol on the brush it will remove rust and the varnish that builds up in just a few rounds. I use a 9mm in the .020 and a 270 brush in my 010 cox motors. going larger I use a 10ga in the .15 and .09, .074 Cox motors. If you have a lathe you can chuck up even larger cylinders in the lathe and then using your brush or again with a bore brush chucked into your cordless drill  you can scrub the bore of the engines cylinders. I also have bronze wool at times listed when I can find it. I use 00 size bronze wool. I can split a wooden dowel rod with a saw blade by threading the bronze wool through the slit I have cut in the dowel i can then insert the dowel into the cylinder so that a close fit will help scrub the bore to clean it. The best dray