I have an O.S. 120AX "Special" engine, plus O.S. muffler and adapters for sale. The engine is actually better than new...as break-in run and initial tuning has been completed. The engine has never flown, and has only been bench tested, to collect data for a magazine article.
The 120AX Special is a higher performance version of the engine compared to the standard 120AX, featuring a billet velocity stack, specially machined crankshaft for increased intake timing, and a special phosphor bronze head gasket for higher compression. More details can be found here http://osengines.com/engines/osmg0652.html
The 120AX Special does not include a muffler, however I do have a new O.S. Power Box muffler included, which is made for the 120AX. I also have a straight extension adapter, and an angle adapter, all of which are included. Here is a run-down of the prices new for these items:
O.S. 120AX Special, $330