I have four things to say,
1 Welcome to AS plenty of info and smart fellas here
2 OEM Stihl can be had at dealer but expensive
3 Do a search,upper right of that green bar,click on it type in 028,get your favorite drink and sit back for alot of reading.
4 Check out Triggers post #4 he gives link to aftermarket piston and cyl. http://www.arboristsite.com/showthread.php?t=83031&highlight=028 Here is link. I screwed this up is was supposed to be thread but its link to parts. Sorry