The 116i and 120i mufflers etc are all the same. The difference the is obviously noticable ONLY when you put them side by side is the actual cylinder/barrel & fins arrangement, otherwise they look identical. There shouild be a I.D tag below the clutch, the later ones have a 2 digit number for the year of manufacture. Later the better in my view. (has torx head bolts/screws)
The 120si has a chrome plated bore/barrel made by MAHLE, the 116i has Niksal bore/barrel. The 120si has 'MAHLE' cast on the outside at the square base of the barrel(should be able to see it) I think on the chain side. The
116si's that I have, don't have manufacturer cast ID's that I could identify. Either one are very good,
Looking at both barrels, I'd go far as to say the porting on the 116si is a bit wilder but the 120si has a lotta torque and revs.
Make sure the saw has got a choke lever that rotates 180 degress. If it pulls out or has a primer bulb, it is not an "si".

Peter