Ya, topcylinder is the end that goes back in towards the exhaust opening. Top is top. The do dad with holes you can drill those larger, and add more to the bottom. I tend to still use my screens as I used to be a volunteer rural firefighter of the outdoors nasty kind, so I think spark arrestors are worth a slight performance hit. If a saw has one when I get it, I leave it there. I'm not in a race saw competition nor will taking five minutes longer (whatever) to cut a load of wood hurt me any economically, so I leave them in. I know that is
"leaving something on the table" performance wise, but I don't care either. Just opening it up, but still leaving the screen, makes a noticeable diff in saws.
You can keep the screens clean by burning them with a propane torch. You can wire brush out the inside of the muffler, and the parts, or however you want to do it, soak in some nasty solvent, whatever, to decarbonize it. I just hit mine with a round wire brush in the cordless drill, knock off most of the nasty stuff.
Modding the muff case, look around the site for pics, lot of ways to do it. Some guys do amazing jobs with tubes and brazing, others just open up the existing slots or holes, etc. Salt to taste. Rule of thumb is no more then around 80% or 3/4s say of your exhaust opening in the cylinder. Two strokes need a bit of "back pressure" to function correctly. The race saw guys have tuned pipes that do it perfectly, but those are way clunky for general use. (now someone will chime in how they use their piped alkynitro burning hotsaw every day for the last 50-75 years to make more money out in the woods) HAHAHAHA
Always find the IPL for your particular saw before you start working on it, it will show all the parts and how it all fits together.