The exhaust is still OEM. Although I've spent my life boosting the performance of two-stroke bikes, I only realized with this issue that I could do the same for a saw. There was a time when I built log frames and I modified the exhaust so that it would blow the sawdust away from the work area. Back then, visibility was the main concern, not power.
2 stroke bikes usually come with an expansion chamber or at least a pseudo expansion chamber muffler, where as saws never come with an expansion chamber, consequently, the muf will be a limiting factor as they are deficient in scavenging exhaust gases. I have an 028 as well and way back when, it got used pretty hard but in the last decade and a half at least, it has lived a easy life.
I bought it new in 67 or there about as well, bought it as well as my 090 and 075 all at the same time actually and first thing I did was open up the muff and adjust the jets accordingly. For years it's lived on a steady diet of canned fuel, either Tru-Fuel or Echo Red Armor (I own a couple Echo saws that are pretty new as well).
I really did nothing other than the muff as it's always pulled a 20" bar with full tooth chipper for me without hesitation but it's gonna need crank seals this year I suspect, it's time because of age. I did replace the impulse hose and fuel hose a while ago (OEM parts, not aftermarket).
Mine runs like a scalded dog, it's an RPM saw btw and Stihl don't build them like that toady. It's 100% alloy but the top shroud and mine has the heated handle as well.
Like my 090 and 075, I'll never sell it. They all represent an era gone by for Stihl.
One nice thing about using canned gas in it is I don't get ANY carbon buildup in the exhaust parts and last time I peeked in, the piston was totally free of any damage as was the cylinder itself.
Don't buy a new Stihl, keep the 028, it's a damn good saw, Mine is designated 028 WB by the way...