The MDS 78 is not a high rpm engine. The minimum prop would be a 13-6. You might try something up to 14-6? The engine has very mild timing. The engine will not perform with any more power than a 61 with a 12-6 prop.

I don't know how close it would come to fitting but the Tower 75 muffler might be a good candidate? It is a tuned canister type muffler.

The GMS 76 is engine is almost the same but the muffler on the 76 is not a tuned cavity type.

The holes are 41mm o/c and it costs $18.99 USD

http://www2.towerhobbies.com/cgi-bin...?&I=LXVH43&P=Z