Modding a saw is a pretty universal in approach, from saw to saw. The variations come from what exact specifics work on specific saws.... What I am saying is look at the techniques and approaches of other saws and base your mods off of those. I have not personally seen any threads specifically targetting the 520i.
First things first, figure out a way to open up your muffler some, a muffler mod. Usually the most direct, clean way it to find the existing muffler exhaust port and increase the size and decrease flow restrictions. If there is not enough room, you will need to do a bit more custom work by way to installing an additional port or drilling some strategic holes. After you get the muffler opened up, you MUST open up the H screw on your carb to allow more fuel into the cylinder to accomdate the increased airflow.
The next step will be your cylinder. You will want to figure out how to measure the "squish" or comustion chamber clearance and then make gasket or clinder base adjustments to get that number in the range of .020".
Typically, for as much as you lower the cylinder to get the desired squish, you will need to raise the exhaust port to keep the flow timing ideal. Both the intake and exhaust can be widened a little bit to increase airflow. If you really want to get into it, you can also shave some metal off of areas in the piston, depending on piston design.
Your best bet is to search all sorts of different porting and modding threads for all different brands, types, and sizes of saws and get a feel for what you are trying to accomplish, then get to making some changes. The muffler mod will be the best bang for the buck and is such a reliable, doable mod that anyone can get good results from it.
