OS A3 plus should be hot enough from what I've read. Horizon makes a $59.99 muffler for it now, from Jett engineering.
http://www.horizonhobby.com/Products...x?ProdID=EVOM1

Of course the same muffler fits many engines of the same size with the same spacing, Thunder Tiger, OS, Magnum, ASP, Megatech, Evo, etc.