Hi!
The reason for buying a 120 instead of a 160 is mainly the weight. For the O.S AX 120 and the O.S 160 FX, there is a weight penalty of 335 gram or 12 oz when going from 120 to 160 (included muffler)

Also the standard muffler for those two engines are very different. O.S "power box" muffler is easy to fit inside a cowl. However, I have not any weight data on a pitts style muffler for the 160.