I've just looked at my 28 and 31 Ryobi's side by side. The 28cc has the earlier fins as described by Bob above and the earlier crankcase as described by me. The 31cc has the later fins and the later crankcase. I have no information on the order in which the changes were made during production and whether they were in any way synchronized with the change from 28cc to 31cc. So if one really did want to know whether it was a 28 or 31cc then measuring probably is the only way.

As for the muffler, either Carr Precision or Wacker engines can supply. They also supply all the other parts needed to convert.

Maninder.